Hi, I work at an apple-authorized service provider. Your Macbook pro unibody is interesting, as to replace the keyboard, one has to disassemble it completely, and put all the parts in a new top case. The non-unibody macbook pros, you can actually just swap out the keyboards (though this isn't recommended by Apple: they only sell the full top case).
The biggest issue is making sure the keys fit. We had one customer who provided some keycaps to swap out on his, but one was the wrong size. Ofcourse, all the alphanumeric keys should be the same size (no guarantees though, try to get ones from the same model mackbook pro). The biggest issue is normally for the function keys at the top.
And, if you want a 100% new set, from apple, they will sell a new top case for something like $200. My work would also charge you $100 in labor to install it.