Its is fairly rare and the only place I have seen and caught it was in the Safari Zone. Be care it flees very quickly.
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Use Theif On A Wild Chansey
Shroomish is not available to be caught in the wild in Pokemon FireRed, it must be traded from Ruby or Sapphire.
Wild Chansey
Actually, I think that is a Johto region pokemon, and seeing as Firered is a 1st gen remake, which is set in Kanto, you can't find it in the wild. You might be able to trade for it.
I do not know what Pokemon you like, so try going to www.serebii.net and find the Pokemon you like, then look where they appear on FireRed.