they tend to cost around 2 pounds for each key cap so if you need a few might be worth pricing a full keyboard up, providing the fixing (the bit underneath the key usually 2 bits of plastic) is intact its a simple case of popping them on, if the fixing is damaged or missing it can be quite fiddly to put back on, best to pop of an ajacent keycap to see how it works, also check you get the correct fixings, the same model of laptop can have different types.