There's a bunch of ways you can find your SID:
Use IR-GTS to upload a .pkm file (of one of your own caught pokes) to your computer, and open it in Pokesav.
Trade to someone who can do the above, and get them to tell you.
Capture a random shiny poke, and use the calculator in RNG Reporter to find it out.
Capture ~6 or 7 of the shiny gyarados from the lake of rage, and use the calculator in RNG Reporter to find it out.
Choose your favourite.