Thanks. I think it is kind of a racket that air purifiers don't make any that use regular furnace filter sizes. I got several purifiers, but the one I put in the cat room, the replacement filter (if I get the manufacturer's replacement filter) is about $35 and the quality really isn't any better than the typical furnace filter.
As a work around though (since the size of the replacement) is about a 16" x 14" (I'm doing this by memory so the numbers are probably not exact), I get the cheapo furnace filters that are about 16" x 20" and have good garden clippers to cut them to the proper size
. That's $35 vs about $3 for a filter replacement.