Can't help you on a supplier.

What are the symptoms?

If the problem is starving for fuel, it should run on PRIME OK.  You will no longer have a RESERVE, but that's not a big deal if you keep track of distance between fill ups.  

If the problem is leaking into the carb when stopped, maybe you could find a small valve that you could put between the petcock and carb and shut off the fuel manually.  You could even pull the fuel line off the carb when stopped and then plug the line from the tank with something like a golf tee.  

Not the ideal solutions, but as they say, "necessity is the mother of invention."

Thanks for replying - it´s the old story of a bike (been standing for years) having a clogged carb and a failing petcock. Even on PRI she cuts out, backfires excessively and dies on deceleration. I was going to clean the carb and swap the petcock in one go and have it over and done with. Before that parade

Thanks for the suggestions, guys. I just ordered one stateside, let´s see if the USPS is up to the job . It should be here by the end of the week, best case. Meanwhile, we drained the tank and attempted to remove it. Bloody impossible, it turned out. The petcock is in the way, so there was no way of bringing the tank far back enough to free it without removing the petcock. Idiotic design..so, while en route, we might as well "slot mod" the spark plug cover for easy access in the future.

BTW, the vacuum thing still works. I dismantled it and applied suction, and it moves inward ca 5 mm. Lord knows though, whether the vacuum in the carb is all that strong. It´s a suspect, an accident waiting to happen, and will be swapped anyway.

Today, some basic tool knowledge was transferred to the young lady, who actually did all the removal stuff by herself. Now implanting the good habit of putting things that belong together in small boxes with tape labels. And taking pics of everything.