I run into 4x5 plate holders all the time. With the PHSNE Photographica show in Wakefield, MA coming up on April 30/ May 1, I expect I will see dozens of them. They seem to come in waves, though, as collector/dealers empty out their bins of old stuff. While on APUG we have quite a few people who might use them, this is a cross section of international photo nuts (I include myself as a complete nut, so please don't take offense.) Overall, the number of potential users is certainly lower than the number who would want film holders - or at least holders that are converted, and that is in itself not a huge number. Keep looking at any photo show you can find or at antique dealers, where they are often described as "good for display."