try the hardware store for a 1/8 brass ball valve or what ever you can find that will fit. One problem is I think the threads are metric on the tank so you will need to check if your close to a cycle shop they should have a tank valve.
J&P cycles have a online catalog and list fuel valves check them out.
I has my Dax kit fuel valve do the same thing I was removing the fuel line and got a little rough but I had an extra fuel valve that is brass. I layed the bike on its side and got the dutchman out of the tank installed the new valve and was back in business quick hardly lost a drop of the high dollar perfume we all refer to as fuel.