Look under plastics in your local yellow pages. You CAN find places on the web, but shipping is cost prohibitive due to size constraints.
I have found that I like tinted glass better for putting in front of my monitor because plexi scratches when it gets dusty and you clean it. Glass is much more durable and cleans better. You can find tinted glass at almost all glass companies for the same or less then tinted plexi and they will cut it for you.
Good luck~
Kev