Memphis Equipment has them, at least in Chambersburg they had them in the store. The kits are ~$35-40 each. Steep, but when I looked around no one else had anything close for a reasonable price. Each kit came with one petcock, a barbed connection hose clamps and face bushings. If you want 2 petcocks, for the inlet and outlet you will need 2 kits. The price is high, but it was either that or rig something up that will "make do".
I bought 2 kits and put it on my truck beacuse I wanted to make sure I could isolate the heater if a hose ever broke. Not sure if the antifreeze would back flow out, but I wasn't willing to take the chance on the road.
If you need the kit number PM me and i'll dig up the bag they came in, but I am sure they will be able to help you out.