I've only dealt with the well-known kart parts suppliers like BMIKarts.com, MFGSupply.com, GoPowerSports.com, etc., and with a fair amount of ebay parts which is mainly all of my hotrodding stuff from my performance guy.
I do recommend not going with direct-mount sprocket (hub bore same as axle, slides on from axle's end). I believe in sprocket hubs, that also slide on from axle's end, but just once. After that you only use split sprockets, 2-piece sprockets of your choice of size and tooth count, that all bolt directly to the sprocket hub with 6 bolts, change out in 5 minutes flat.
Splits are much cheaper than direct sprockets too, after buying the initial hub. Main benefit for me personally is never having to remove wheel, bearing, maybe brakes, all just to change a direct sprocket. And put it all back on.