Super SmartJoy is a snes to usb adapter. $18.90 US at Liksang.com.
This is completly compatible with all emulators, all you need is an snes controller. I use this for all of my emulators, snes controllers I feel is comfertable in the hands and has enough buttons for just about every game out there.
SmartJoy puts out usb adapters for just about every console controller on the market.
If you truely want a retro feel you can purchase an NES controller converted to usb at retrozone.com. they also have the fourscore, nes advantage, snes, genesis, virtual boy, atari, pc engine, intellivision, colecovision, saturn, 3do and vectrex controller usb adapters and kits. I have the nes usb pad and works perfect.
If you have the knowlage or feel brave you can download instuctions to convert it yourself.