50cc.nl (in the Netherlands) also sells them, and a host of other parts...but since they're in Europe, they're usually only worth dealing with if you're ordering a ton of parts. The stock belt size for the Bravo / Grande is very tough to find, or so I've been told, in auto stores.
Belt slippage like you're experiencing is quite common and among one of my pet peeves with the Vespa bikes. Someone did say you could drill out the rear engine mount hole so that you can slide it forward a bit more and tighten up the belt tension a bit (this is how you adjust belt tension on the non-variator models like the Ciao and the Kinetic). So that might be worth a shot too.