You can get one by fishing in Vermilion City. It is somewhat rare and you need the Super Rod to be able to get it :)
Use a good rod.
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In Pokemon firered you can't, but in Pokemon leafgreen you can get it then trade it to Pokemon firered.
lombrea, evee, poliwhirl, shellder, and staryu. some of these can only be found in firered and leafgreen so you will need to trade. that's all I can think of.
You can get over 151 pokemons. Your starter off Pokemon will/could be Bulbasaur, Squirtle, or Charmander.
You can fish a staryu in lilycove city.
For staryu to evolve you have to use a water stone