I think you'll have to take it somewhere to get the keys, or you could just purchase the keys then put them on your laptop. Stores like BestBuy and Wal-mart should be have them.
Laptop KeysThis is actually a good website to use. It has instructional videos, and pretty much every single brand keys You can buy them online and they have a link to a video to show you how to put them on. But you gotta know what kind of keys you have and which laptop.
They're definitely replaceable, I know that much. I've gotten a replacement for my 'J' key once, and it's impossible to tell it even came out, but I don't remember where. If you really don't want to pay, I advise doing as Graham said and stick some sort of construction paper onto the key spots.