Local sign companies and suppliers usually have it. They usually stock #3. However getting these jerks to sell it to you is another story. Most "sign companies" will not sell plastic or fabrication meterials to end users, they are downright rude about it in most cases. The local "acrylic" vendors usually sell to sign companies. In most cities will not even talk to you unless you can show them that you are a sign company.
How do I know this? Well after calling a dozen or so suppliers and sign companies here in pittsburgh and getting "get lost" type of answers, I decided to call companies in several major cities or markets (just to see if this was a local thing). I found out that it is an "industry" thing. BTW I offered to open a business account with my FED EIN and State tax ID, still got the same answer.
TAP plastics has locations in most cities, and they will sell to you in most cases. See if you can find one.
Good luck....