On the cheap side of things you could always just pick up a $5 USB gamepad and hack it. Though for reliabilty I'd go with one of the off-the-shelf options like a KeyWiz, iPac, or Kade. For a simple setup like yours, I'd go with a Kade. If you don't mind soldering then the DIY kit is only like $22. If you don't want the cool-guy interface board, you can find 16k minimus boards that will support the Kade Loader software for as cheap as around $6.