The last company I worked for used to buy felt in sheet form for the remanufacture of a particular type of filter and it is still used as a filter medium. The spec. pretty much matched that of the wheel felts.
I would suggest 'googling' industrial filter manufacturers or industrial suppliers as a potential source. I was considering the possibilities of replacing the felts with oil seals but haven't looked into this in any detail yet.(I'll do that when I start the overhaul of my M20 later in the spring).I think Hans has spent some time on this subject..he may be able to comment..Ian
email (option): julie@wright52.plus.com
Yes, I did some digging, but the problem was that the company that has the hard felt needed in the right thickness (about 1/4") only deals with other companies and not with private persons. I could order through such third party but that will about double the price.... 
However, I'll try again to get a square foot or so of the stuff
email (option): viaconsu # planet dot nl