just do a search, lots of places sell them on the interent. Radio Shack has one, although I did not have good experience with it. Of course it was only 10 bucks, and I often find the old saying "you get what you pay for" is true.
I'm not saying the expensive ones work better. I dont see the point in spending 50-80 dollars on a converter when for a little more I can buy a decent USB joystick. But ive seen plenty of them between 20 and 30 bucks.
*edit
Heck, I remember seeing one company that makes a USB converter to plug your playstation or nintendo gamepads into a PC and get them to work. Go figure. Technology can do wonders.