i buy these off ebay. they're awesome :
Threaded Fuel Shut Off for Metal Gas Tank lawn/Tractor - eBay (item 300351445254 end time Dec-20-10 10:07:09 PST)
the weird thing about the seller though, is he keeps changing the prices. right now they're 9 bucks, but a coupla months ago i bought 5 of them for 20 bucks.
you could probably find the same thing at a lawnmower shop.
like 2door said, you'll need a 1/8" NPT tap.
i've had a few of the stock petcocks that leaked, all of them brand new. nothing like finishing a brand new bike, filling it up with fuel, and watching it pour all out on the ground.
the only thing i use the stock petcock for now is to plug up a custom tank when i seal it.