Take apart your carburator and take out your jet screw (see below for instructions). There should be a number stamped on the screw side (where you'd use a flathead to take it out). That's the number of your jet. Most people seem to prefer 52, though I'm using a 55.
I'm not sure what they cost, since I've never had to order one. Check online or call Handy Bikes (best bet). They shouldn't be more than a few bucks. Moped Junkywar has them online for $6.95 (see <A HREF="http://www.themopedjunkyard.com/dellorto.html"http://www.themopedjunkyard.com/dellorto.html</A>)
To take apart your carburator (these are instructions for a Dellorto, but should be universal, I think), first be sure that the pet cock is off, and unhook the fuel hose from the carburator. Then take out you should be able to take off the carburator rather easily of the manifold. It's OK to leave the throttle cable attached. Next, unscrew the metal bowl off of the carburator (there's two screws there). Now your. Now you should be able to see a white plastic float and the jet screw. Just unscew it out and take a look at it.
I hope this helps.