If you're familiar with C coding you could make your own adapter. ATMega328 for example is about $3 a pop, has 32k EPROM inside for storing code and can be used to convert damn near anything to work on damn near anything. All that's left are PCB, wires, and connectors.
Making PS2 controller work on PCE and implementing the full 6 button support would be easy. There are large arcade style controller that has all the buttons in one area rather than on top and on back like standard controllers.