Two types of air filters were used in the same air cleaner housing. There was a “pleated paper” element that was disposable and replaced about every 10,000 to 12,000 miles. I have never seen one, new or used. The most common type is a breathable (as in “open cell”) foam latex rubber wrapped around a screen form inside the air cleaner. Instructions state it should be immersed in light weight oil and squeezed out to leave a small amount of oil on the foam rubber material.
If you have the screen form, a new piece of foam rubber can be cut to fit. This foam rubber can be used for many years. Just immerse in a solvent tank to clean, then re-oil. I make the foam element from 1/2″ to 3/4″ material. Cut about 1/2″ taller than the screen frame. Cut the circumference to the wrap around length of the oblong circle. Use a hot glue gun to adhere the two ends together. Should be a very slight stretch over the frame. Allow about 1/4″ taller than the frame on top and bottom to seal inside the air cleaned. Immerse in light oil, squeeze out and it is ready to go.
November 2010 update: I have now found “one” paper element type filter. First and only one I have ever seen and of course, I bought it! They were made by AC Delco and have a part number of A146C. I have not ever seen a cross reference to other brands. If you keep your “feelers out”, you might just find one someday also. I have seen several references to “Jetfire”air filter elements for sale on eBay, but all have turned out to be misrepresented. Pic’s of the “real McCoy” attached.”