Go to the head of the stream in Italy.
Pedersoli will probably have the part, or one that will work. Find your model number, look up the part number for your tumbler. I have bought parts from Ped that I had to cross check the P/N from different model guns This can be done using their P/Ns for different models.
Most of their parts now, are CNC machined, and are very good quality.
It takes about 3-4 weeks to get parts shipped direct from Italy, cause they have to go thru customs, and all. Be prepared to spend a little time on Peds sight checking things.
Dave