If you tell me which you're trying to find or evolve, I can give you its exact location or method!
Every Pokémon from Bulbasaur (#001) to Celebi (#251) is obtainable in-game.
If you're aiming for a complete 251 Dex, you'll need to hunt down the Johto legendaries. In this hack, they are often placed in thematic locations:
: Since you have access to Dark and Steel types earlier, consider adding a Houndoom or Scizor to handle Kanto's Psychic-type dominance.