Rob, I believe Crown made the Ratshack ones, the square plate ones.
Crown owns the name "PZM" and when Ratshack started having their own made, they had to call them 'Boundary Mics'. They have them today for thirty bucks. Not bad, either, but not as good as the old ones.
Crown makes a $75 version called the Sound Grabber. It looks like a paddle on a defibrilator. I've heard that's the same element as the old Ratshack PZM. You know how that goes, though.
The Crown 30D for about $325 is their 'high end' mic.
Somewhere on this forum, Fletcher talks about making a PZM with any condenser mic by putting it up close to a piece of glass or something to make a pressure zone.
This would probably be the best way, since you'd have your choice of mics. The electret element in those PZMs is not bad by any stretch, but not the best in the world. But there's a place for everything, good and bad.
Back in my Otari 5050 days, I made a wedge out of two one foot square pieces of plexiglass (perspex) and mounted one PZM on each square. Then we'd hang it over a drumkit. Sounded great. More low end from the larger surface.