I have nearly finished work on a new version of the interface presented here: http://joystick.world3.net.
The original version supports 13 buttons and a USB interface. The new version has dual USB and Playstation interfaces, and supports 20 buttons.
At the moment I just make prototypes on stripboard for fun, but it would be possible to do pre-made ready-to-go boards. For low volumes I’d do stripboard, but if there is enough interest I’d look at doing PCBs.
Likely cost will be around 10-15 ($20-30) plus shipping for each board. Of course you can often hack a pad for less than that, but this is a cleaner solution and much more robust. I’m also interested in swaps for parts or modable sticks.
I can be emailed at firstname.lastname@example.org .