I am always on the lookout for good vendors with reasonable prices. I have been buying my acrylic sheets at the same place for a while now, and thought others might be interested as well.
The dealer is
Delvies Plastics. They provide good service, reasonable shipping, and what I have found to be the very best prices. For example, 1/8" clear cast acrylic is $4 per square foot, with discounts starting on the 2nd piece. 1/4" is $7. Compare these prices to the popular places that charge 2 or 3 or more times that amount.
They also carry colors, transparent, translucent, mirrored, and a variety of thicknesses, as well as tools, solvents, etc. they use lasers to do their cutting, so you don't lose 1/8" off the stated sizes for the saw kerf.
If you need a good source, give them a try.