The ones I know off the top of my head:
-Scizor, Araomtisse & Slurpuff can't be found in the wild. Have to trade for all 3
-Steelix you have one shot at catching in a post-game quest.
-You can't find Slowking in the wild either, but you can evolve the quest Galarian Slowpoke into Galarian Slowking and that'll fill out that part of the dex.
There's a post-game Wild Zone that let's you pick up the Kanto & Z-A starters (at high levels that you can just evolve them twice each) and has a bunch of possible alpha spawns including high-level stuff like Metagross or Tyranitar & the few trade evolutions not mentioned above (Machamp, Alakazam, Gengar), but you'll probably be just as fast just leveling up/evolving normally.