That's just Plum Crazy!
I've lost track. When did this machine come out? Wasn't it in the mid 70's?
Well, it goes to show that most of what happens on eBay is all about the "sellers market". Truly, not everything is a bargain. BUT if you just gotta have it, you can usually find it. Looks like...
I just found one on eBay
Here's the link:
http://cgi.ebay.com/Dome-Motor-HEPA-Filter-for-Tri-Star-TriStar-Vacuums_W0QQitemZ260345611634QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item260345611634&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A1%7C293%3A1%
The Filter-Fresh also has a pleated filter.
Obviously, it's much smaller. Everything I ever heard from others indicated that it restricted the airflow way too much. A few years ago, I found an aftermarket pleated filter on eBay that fits on the motor intake. The air restriction is much less.