Yep, the air filters are super hard to find and I never found one. If height isn't a factor, or originality, here is what I found.
1. There is some Mercedes paper filter that has basically no "plastisol seal" and fits...maybe a mid 80's something Mercedes? I can't remember.
2. I think the best way is to get a six pack K&N, trim it down and hold it together with bread tie wires & glue
3. If you can get your hands on some plastisol, wire mesh and filter media, you can actually use the air cleaner itself as a mold and make a custom air filter. I did that once with an oddball air cleaner I had on an old Ford and I used yellow plastisol for a unique look.