If it says Weber then it's a Weber (Italian) design but made in Poland. It's possible therefore that a gasket kit for an Italian Weber carburettor of the correct type (but different Polish No.) could be used.
Have you stripped the carb to compare the gasket/s required with what is available for the Italian Weber carbs?
Here are pics of 2 gasket sets for an Italian Weber 28 IMB carb. which is fitted on the Fiat 126 594 and 652cc engines. Possibly these might suit?
If not, and you can't find anything else suitable on the Classic Carbs website, maybe contact them direct and enquire if they can supply a suitable gasket set (they might already know what suits a Polish Weber carb. having encountered this problem before).
As a Fiat (and Weber) trained Technician back in the 1970's, I'd suggest you try to get a service kit that includes a new float valve and needle unit - this is often worn on older cars and can cause many problems including fuel leakage (gaskets rarely leak unless split, fasteners have been overtightened or joint surfaces are no longer flat).
Edit : I don't know what happened with the above links, they were working, now they're giving a 404 error
. Anyway try the Classic Carbs website and input Weber 28 IMB in the gasket section and see what comes up
