you know how on the guitar you can just look up tabs and you can play a song. but how are you supposed to get keyboard music? are me and my friend just supposed to buy EVERY song we play or what?
You can easily transcribe guitar tablature into standard notation for piano/keyboard.
Basically, yes
Or learn it by ear
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find out what the chords are and play those on the piano, maybe just in the bassline, then throw in some stuff over it with your right hand.

or, just play a bass note with your left and the chord in the right

or something =p

basically the chords are easy stuff.
You can transcribe tabs into standard musical notation, and then play it on the piano - if you gotz teh skills. You can dig up materials on sight reading for guitars on the internet easily; would just take a bit of time to figure everything out, and a lot of trial and error to do it right.

edit: oops, did you mean just "where do I find notations for keyboard/piano music", and it had nothing to do with guitars? sorry if so, I don't really know but I'm pretty sure you can find notations for keyboard pieces on the internet just as you look up tabs for guitars. Google it, or something.
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yeah, the thing about sheet music is that it's owned by The Man. tabs are open sourced, and thus technically not copyright violations because they are "interpretations" of songs. however, if by any chance you're just looking for classical music for keyboard, you can find it for free here, because most classical music is in the public domain: http://www.sheetmusicarchive.net/