Jackshaft, 30 series driven, belt. You can try GoKart Supply, BMI, OMB Warehouse, Go Power Sports, AZUSA, MFG Supply or any of the dozens of other parts suppliers. Or you could go the cheap way out and go to the jungle store and get a cheap 30 series TAV II CVT kit and then a Genuine Comet Belt and save a whole bunch of green for gas and oil. Except for the belts the Jungle stuff is every bit as good as the Comet stuff.
When it comes to belts though only the Comet belts are worth buying. You will melt 10 or more cheap belts before the Comet is worn even a little.