You can always get a cheap (remember that word, *cheap*) Jamma harness anywhere. It's a toss up as to how the quality is going to be.
One thing to keep in mind, is to make sure that if you get a *cheap* jamma harness.. make sure that, with modification, all the ground pins have their own separate grounds, AND ****that the +5v pins have their own 18awg wire*** (same thing goes with -5v and +12v).
Most of the chinese ones you find on ebay have +5v tied up with a thick insulated but THIN gauge wire.. this is bad. So are the thin metal pins they use in the jamma connector, as well as the connectors themselves can chip easily or break the connector pins free leave them out of place
I'm not sure how the quality is from other sources like twisted quarter and what not, maybe they're good.
But hands down, the best Jamma harness I've used, and the most consistently built as far as quality - Bob Roberts. You really cannot get any better - it's money well spent and a big headache you do NOT have to deal with