I took a chance on the officially licensed Retro-Bit six-button arcade pad when I needed to pick up a second six-button controller recently. I'm glad I did. I don't know how they did it, but it's almost identical to the original. It looks the same, it feels the same, and I've had absolutely no issues with it at all. If you don't want to pay the ridiculous prices for an original, this is worth picking up.

http://retro-bit.com/sega-collaboration