There are a very few, highly expensive repro filters around from a batch made years ago. Not wanting a $500 or $1000 air filter I bought a K&N for around $110. That was the only replacement I could find. I cleaned my old one (not sure of its origin) with carb spray cleaner and stored it away in a box - just because I couldn’t stand to throw something that rare and scarce away.
May take a bit of searching, but if you have the dimensions, you might find it here.