You can try here:
My experience is that most clear polycarbonites are poor UV performers. However Lexan is one of the better ones.
I did a prison renovation project a few years back and had to design an exterior no contact barrier. I used a proprietary poly sheet system. The manufacture I dealt with was Rplastics. Look them up, I'm sure they can help. As a matter fact, give them a call. Their sales persons are pretty technical. Atleast the two I talked to were. As I recall, they had a huge collection of plastics shapes and types.