I just got a Jamma harness from jammaboards dot com, I've ordered from them in the past and been happy.. They ship fast (ordered friday, came today priority post).
I'm a little let down with the harness I got, though. -5V isn't wired on it (need it for the CPS2 I bought it for), service switch isn't wired, not all the ground pins are wired, there's no polarizing key, both audio lines are the same color (white) and it's nice to know which is hot and which is ground. All the quick-disconnects for the buttons are teensy (they do mention this on the site) - but I always planned on clipping them and re-crimping my own.
I can make do with it and all, it's just a bit of a pisser.
On the plus side, I only ordered a replacement battery for CPS2, and they sent me the "kit", so yay, free torx bit for me. Chicks dig torx bits.
Anyone else buy stuff from these guys? I'm not against them, I said before, they ship fast, and their fingerboards are top-notch. Just sucks that they sell such crappy harnesses.