Buying from a local acrylic supplier has its inherent value but if you don't have one in your area and you need to order, I highly recommend delviesplastics.com. I order all my acrylic stuff from them. In a pinch, and I need something that delvies doesn't carry, I order from tapplastics.com and/or usplastics.com.
delvies has by far the best prices and are very friendly.