The following are online vendors which are good resources for finding things that may not be easily found locally:
This site offers a wide range of plastics, from sheets to tubes, acrylic to polycarb, and much more.
Tel: 800-537-9724 or Live Chat on their site
There is a huge selection on this site, but it might be a little pricey.
Large selection of acrylic, including sheets, mirrors, rods, balls, and more.
E Street Plastics
Decent selection of acrylic sheets, tubes, rods, mirror, and more. Save money on shipping by buying more than 1 piece at a time.