I went to the local honda dealer to buy some main jets for the 50 (going to rejet after filter/exhaust/timing mods) and they didnt even have any jets in stock.... they only sell the hex-head keihin jets.
I usually order mine from classic honda. I always get them quickly and you can order just the size(s) you need, or they have them in sets of four. Check out their website www.hondaminitrail.com