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.
Decent selection of acrylic sheets, tubes, rods, mirror, and more. Save money on shipping by buying more than 1 piece at a time.