Is there a resource listing what Mega Pokemon are available before facing the elite 4 in XY and ORAS? I don't mean just Megastones because apparently some Megastones are in games where the Pokemon you can mega isn't available.
No but with a bit of research you can figure it out on your own. But here is what I got:
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In XY, disregarding the Torchic Event, there are 9 Mega Stones available pre-Elite Four, though you'd probably only will get 6 (or, if you pick Charmander from Professor Sycamore, 7) of them:
First is for the Kanto Starters which you can't miss as its a story event. After choosing a Kanto Starter from Professor Sycamore who'll also give you their final evo's Mega Stone(s). Now while the other Mega Stones are then sold in the Stone Emporium (at first for 1 million P, but you can lower it all the way down to 10k P the more
Stylish you become), to get the other Kanto Starters you'd have to Trade them in from another game.
* Bulbasaur & Venusaurite
* Charmander & Charizardite X + Charizardite Y
* Squirtle & Blastoisinite
Then going in order:
* Aerodactylite & Aerodactyl (After chasing Team Flare out of Glittering Cave and talking with the scientist from the Fossil Lab (the one who gives you either a Jaw or Sail Fossil), if you go to the back of the Fossil Lab in Ambrette Town the scientist will also give you the Aerodactylite. To get an Aerodactyl you'll need to use Rock Smash on rocks in Glittering Cave until you find Old Amber which you can revive into one at the Fossil Lab)
* Lucarionite & Lucario (Can't miss story event. After defeating Korrina atop the Tower of Mastery in Shalour City she'll give you the Lucario with Lucarionite you used to battle her to obtain the Mega Ring, thus also unlocking the ability to Mega Evolve)
* Ampharosite & Mareep (On one of the small islands in Azure Bay an old man will give you Ampharosite. On the route before, Route 12, you can get Mareep in Horde Encounters)
* Gengarite & Haunter (In Laverre City there's a Hex Maniac who'll give you the Gengarite if you have registered any member of the Gastly family in your Pokedex (just having seen them is enough, and being Haunter is the only one encounterable around that time it'll probably be it). Getting the Gengar is the tricky part as you'd need to do a to-and-back trade with another game. But first you can catch Haunter on Route 14, Route 19, Frost Cavern, or Victory Road (all rare encounters BTW))
* Abomasite & Snover/Abomasnow (Don't remember if this is part of the story event or not, but after rescueing the Abomasnow in Frost Cavern from Team Flare, speaking with it again will give you the Abomasite. You can commonly find Snover (who are one or two levels from evolving) and very rarely a Abomasnow while digging through the snow on Route 17)
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For ORAS, there's a lot you can find before the Elite Four. However some of them you'll probably very likely not get as they require a lot of work. First, trying to go in order as best I can remember:
* Alakazite & Abra (Bought in Slateport Market. Abra can be caught in Granite Cave; like with Gengar in XY you'll need another game to trade to-and-back for Alakazam)
* Manectite & Electrike (Hidden next to Beauty Melissa on north end of Cycling Road on Route 110. Electrike can be found on Route 110 & 118)
* Lopunnite & Buneary (After defeating Wattson and talking to him in the center of Mauville City, go up the Mauville Hills apartments where you'll find a guy in the suit outside an apartment. Interact with him and he'll be demanding rent patment from the person in the apartment, both will then go inside. Leave Mauville Hills and come back and you'll find the loan shark again outside of the apartment and he'll give you the Lopunnite. Buneary will start appearing in the Safari Zone after the Legendary Battle in Sootopolis. Remember, Buneary evolves
through high Friendship... and Buneary starts with 0 Friendship)
* Mawilite & Mawile (Technically Verdanturf, but its a part of it you can only access via a path southwest on Route 117.
Mawile is a Omega Ruby Exclusive found in Granite Cave, Cave of Origin, & Victory Road)
* Pidgeotite & Pidgeotto (In Verdanturf there's a little girl who's missing her Shroomish, saying it went missing near a sign. Read the sign near the entrance and Shroomish will come out from behind it and return to the girl. Talk with the girl and she'll give you an Intriguing Stone. Now go to Rustboro City and talk with Mr. Stone who'll look at the Stone and reveal it to be Pidgeotite. Pidgeotto will start appearing in the Safari Zone after the Legendary Battle in Sootopolis)
* Aggronite & Aron/Lairon (Given by Wanda's boyfriend after clearing the rocks with Rock Smash in Rusturf Tunnel. Aron is found in Granite Cave or in Horde Encounters on Victory Road; Lairon can be found on Victory Road)
* Steelixite & Onix (Stone in Granite Cave's basement; requires a Mach Bike to climb up muddy slope. Onix will start appearing in Granite Cave after the Legendary Battle in Sootopolis or in either the
Mirage Cave near Fortree City or Route 129. To evolve it it'll need to be holding a Metal Coat and traded to-and-from another game; there's a Metal Coat in New Mauville or get it from a Wild Magnemite, Skarmory, or Bronzor which have a 5% chance of holding one)
* Sceptilite/Blazikenite/Swampertite & Starter (Story event. You'll meet Steven on Route 120 and he'll give you your Starter's Mega Stone after battling the Kecleon that was hiding on the bridge. After the Legendary Battle in Sootopolis an old man that sell stones will appear on Route 114 and will sell the other two Starters Mega Stones, however to have the Pokemon you'll need to trade from another game)
* Garchompite & Gible (Stone given by Aarune in Fortree City's Secret Base Guild after collecting 1,000 flags from other Secret Bases. Good luck. Here's something easier: Gible will start appearing on Route 111 after the Legendary Battle in Sootopolis)
* Absolite & Absol (Stone is in Safari Zone; requires Acro Bike to ride on & jump between thin rails. Absol can be caught on Route 120)
*
Medichamite & Meditite/Medicham (Stone found next to the shrine on the fourth floor. Meditite is also found on Mt. Pyre; Medicham are also on Victory Road)
* Bannetite & Shuppet (Stone found near southeast corner gravestone on summit. Shuppet is also found on Mt. Pyre as well as Route 121 & 123)
* Gyaradosite & Magikarp/Gyarados (Give the Poochyena at the 123 Go Fish shop a scratch and it'll give you the Stone. I'm not going to tell you where to find Magikarp; the region is 78% water, figure it out. Though I will tell you in Sootopolis you have a decent chance of fishing up a Gyarados)
* Altarianite & Swablu - (Show the Collector outside the Contest Spectacular Trainer Fan Club an Altaria. Swablu are found on Route 114, Route 115, Soaring in the Sky, and as a Horde Encounter on Sky Pillar)
* Glalite & Snorunt (In the Ice Room of Shoal Cave's basement only accesible during low tide. Snorunt are also in the Ice Room)
* Heracronite & Heracross (On an island on Route 127. Heracross is in the Safari Zone)
* Sablenite & Sableye (Near the Cave of Origin's entrance in Sootopolis City.
Sableye is a Alpha Sapphire Exclusive found in Granite Cave, Cave of Origin, & Victory Road)
* Pinsirite & Pinsir (In a hidden area on Route 124 that requires Dive to access. Pinsir is in the Safari Zone)
* Beedrillite & Kakuna (Sea Mauville's Storage Hold; requires Dive. Kakuna will start appearing in the Safari Zone after the Legendary Battle in Sootopolis)
* Red Orb & Groudon/Blue Orb & Kyogre (Story event. After catching or defeating the mascot Legendary of your version, In Omega Ruby you'll get the Red Orb from Maxie and in Alpha Sapphire you'll get the Blue Orb from Archie. The other color Orb is available post game on Mt. Pyre)
After the Legendary Battle in Sootopolis City some more Mega Stones will appear around Hoenn... but of these only ONE has a Pokemon you can even get in ORAS without trading:
* Aerodactylite & Aerodactyl (Stone in Meteor Falls, a few ledges below Zinnia's grandma; Requires Surf & Waterfall. To get an Old Amber you have to Rock Smash rocks in Mirage Spots, then take it over to the Devon Corporation to revive it into Aerodactyl)
And for completion sake, here are all the other Mega Stones you can get but the Pokemon aren't available without trading them in from another game:
* Abomasite (Route 123)
* Ampharosite (New Mauville)
* Houndoomite (Lavaridge Town)
* Kangaskhanite (Pacifidlog Town)
* Mewtwonite Y (Pokemon League)
* Mewtwonite X (Littleroot Town)
* Scizorite (Petalburg Woods)
* Tyranitarite (Jagged Pass; Requires Acro Bike)
* Venusaurite (Route 119)
Finally, this is technically available as soon as you get to a Contest Hall:
* Lucarionite (From Chaz after defeating Lisia in a Pokemon Contest Spectacular, which means you'd have to get to Master Rank)