you can sand or cut down a faucet washer to the required diameter/thickness to make a rubber washer. I'd go to a real hardware or plumbing store. YOu can also try the same thing with a fiber or plastic friction washer. Hardware store, bottled gas dealers (CO2 washers) etc. Fiber washers are also common in carburetor rebuild kits. Kits cover numerous models and have leftover parts. Think Lawn & garden service shop or car racing shops. Extra parts are the kind of things mechanics have a drawer full of.

I myself would probably cut one out of rubber gasket material I have on the shelf. A .44 mag casing to punch the OD and a .22 to punch the ID.

https://www.lawsonproducts.com/Fiber...s/FS120030.lpc

T&S 001088-45 Seat Washer for Big-Flo Faucets