I haven't found any online sellers of anti-reflective acrylic. I can get it locally, but I live in New York. I pay about $60 for an acrylic Optium Museum 16 x 20 light, and $50 for a glass Museum 16 x 20 light.
The other thing in short supply is unvarnished, plain, good quality wood frame parts. When I had space for a small workshop I made my own from oak. Nowadays, to keep my prices down, I have to compromise. I buy ready-varnished wood parts and sand the varnish off. Maybe I should go into business selling the kind of frames I like, but nobody else does. That's my usual business model - sell something that isn't available elsewhere, but for which the only demand is from me.
Best,
Helen