Well I find a Ralts is a good Pokemon to use. They are psychic types and found, well, at the spot where Wally catches his, East of Petalburg. I found mine to be a valuable asset.

If you want, you could get the Fire/Water/Grass cycle. Shroomish are good grass types, with their effect spore. In RUBY you can catch Seedot, and in Sapphire Lotad are available. Lotad is grass AND water, an interesting combo.

Water types, as well as utilising Lotad's water type, once you pick up the Old Rod in Dewford you can get yourself Magikarp, Tentacool or Goldeen, I think, by fishing in any body of water.

Fire Types are rare until later in the game. Slugma, Torkoal and Vulpix are available, as well as Torchic, of course.

Also, raising an ABRA found in Granite Cave can be good. Theyre nice psy-types.