> Which existing gamepads are available that meet these requirements? Which six button > gamepad can be used to play "Street Fighter II"?
I'm not aware of any 6-button console controller that's wire-per-button. You're at 10 wires just for the buttons and d-pad already then, and SNES/Genesis/PSX/N64/Gamecube/Dreamcast all have fewer pins than that.
As I stated elsewhere in this thread it's possible to make a converter (you'd need to build a small board with a microcontroller, program it to speak the controller's protocol, and output to JAMMA accordingly - no, this is not a trivial thing) with the caveat that the conversion process would introduce a small amount of lag that wouldn't be present with real arcade controls.