Just solder wires directly to the board if you can't find one. It's really quite easy. All you need is four wires soldered, and you can use the ground from the jamma side. If you want stereo sound, you can get that off the kick side as well. But yeah, solder if all else fails.
i may have the harness without wires soldered to it....interested? hit me up with a pm and i will look when i get home
As someone mentioned just make one. Its just an Edge connector.
http://www.jammaboards.com/store/namco-48-pin-kick-harness/prod_145.html
edit: looks like its sold out.
you could make your own though:
http://mouser.com/Search/ProductDetail.aspx?qs=cSZs%2byTnFTRlhJAbsTAzgw==
http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMtVoztFdqDXO%2b6pvnVWRLQv