I've visited the Jets R Us site several times, but I ended up buying from Scrappy Dog. I was buying jets for a 50cc scooter, but they also have horizontal engine stuff:
http://www.scrappydogscooters.com/70...ORIZONTAL.html
I've also purchased Keihin jets OTC from my local Honda dealer. They keep them in a large plastic organizer, and I just match 'em up. I do the same at the Yamaha dealer for Mikuni jets.