Look up “Acrylic Displays” on FB. Original name, I know. But they advertise a lot and go to all the major shows.
A friend of mine got hers from DreamCo Plastics but I don’t know if they are in business anymore.
Also be careful with eBay. I’ve gotten a couple of good items in the past but customer service is a cr*p shoot and it can take a while to get your stuff.