Sorry to hear you are having a hard time finding a larger size of Plexiglas. Have you tried local professional framing companies. I have not had a problem with them supplying Plexiglas for large framing when it was needed. I think the largest I have needed was about 40".
I don't remember exactly what size I got, but Lowe's has fairly large sizes and they will cut it for you if you give them the right measurements.