http://www.ebay.com/itm/Omix-Ada-17704- ... 6e&vxp=mtrThis is the cheapest I can find for a Weber filter, includes shipping unlike the previous link. These private-label filters are for the same 32/36 Weber when it's mounted on a Jeep.
I tried all Weber part numbers I could find in the cross reference sites for Fram, WIX & K&N, none returns a match. I tried the size/shape search for K&N, again no match.
You may have to just continue to buy Weber-specific filters.
OR- I bought an adapter (about $37) to mate any standard 5-1/8" neck after-market air filter housing to the Weber carb, then I can use any brand filter that fits the housing. There's a good bit of initial expense with the adapter and air filter & housing but replacement filters are available anywhere & relatively cheap.