If you want to save a good deal of money, check your local phone book for plastics shops. Some sell their off-cuts at $1-$2 PER POUND rather than the full sheet price. If you're making smaller parts this can save you 90% of the cost of the sheet. Just bring a tape measure and have some idea of how much plastic you will need.
I tend to buy 50 pounds of polycarbonate once a year from
Piedmont Plastics, Inc. - much more than just sheets, rods, tubes which has many locations in the Eastern US.