I want to complete the pokedex and I chose charmander this time I've already caught Mew but I don't know were to find the right trainer to battle
Found a cool diagram that give you the yield of every Pokemon trainer out there.
http://puu.sh/257S
didn't see squirtle specifically but keep in mind you can also use the ditto glitch: http://www.smogon.com/forums/threads/mew-glitch-extended-obtain-any-pokemon.53104/