You Could: A) Find a writer. B) Write it yourself.
If you choose B, please take the time to learn something about writing, and avoid cliches at all costs. If you choose A, good for you, there are plenty of writers to be found who are just dying to write something for someone who will actually make it into a game. I suggest you go to the "Help Wanted" forum and post something in the way of "writer please!" You will probably get a response.
Or did you mean where can you find existing game stories? That I don''t know.
Unless you''re making a pure-technology driven game (like an ultra-advanced, blow-quake-out-of-the-water fps), please, please please please please (am I getting my point across?) PLEASE! make a story first. Don''t just come up with a few gameplay elements and then throw a story in at the last moment. Otherwise, you''re certain to make a game which has absolutely no consistent feel. Good games aren''t an accident: they are a perfect blending of gameplay, story, graphics, sound, music (insert any others I missed here). Games like UT and Quake III are a rarity. Don''t count on being able to come up with some revolutionary style of gameplay that your story-weak game can use as a crutch. This goes out to everyone (not meaning to pick on you Ricardo, you just sparked this). Story matters! Come up with a cool story, put in some cool gameplay, and you''ll have a cool game.