I bought 12 oil filters about a year after getting the R65, it got them from Albuquerque Filterworks, in Albuquerque, New Mexico.
If you bought 12 or more, the filters were $3US each, and free shipping, this is when Purolator ( I think!) was the OEM supplier for BMW motorcycle oil filters ( air filters also).
So I may have gotten the last of the original type filters, didn't buy anymore until 1989, from Competition Accessories.