new keyboards are $12 or less. if you want an ergonomic one, it might go up to $25.
they all now come with a usb or ps/2 connection - and adapters for the other, depending on what it terminates at.
if you have one of the really old school ones with the large circle at the end, like project said, they make adapters for those as well, and they are cheap.
try RadioShack.
the best part is that milk smells really fantastic the older it gets.
I pour milk on everything just for that reason.