I needed to replace just one, and finally took on the challenge to build it myself. No woodworking skills, and limited tools. Took forever, but was quite pleased with the result. Not a project I would care to do again, nor recommend the adventure.
Measure yours (different sizes?) and carry that data with you, with a tape measure. You may find some in good shape at a yard sale. Easy to repaint. Accept slightly larger, which can be trimmed down to fit (not smaller). Junk shops have them. Habitat for Humanity resells them. Craig's List. FreeCycle offers. All should be in a price range of $2 - $10 ea. A little DIY work can save some bucks. Any local retirees with skills who would like to pick up a few bucks? Good luck.

Here's my frame being painted, waiting for glass. Total materials was $40.

Wow I didn't think about purchasing used. I'll look. I need 18 so making them from scratch is pretty much out of the question.
Thanks

Staple up some plastic if need be, but don't be in a rush. The used ones you need are out there, somewhere. Good luck in the hunt and saving big $$$$.