I don't know if they have the "best" prices for acrylic but the place I shop for acrylics and other plastics things is Industrial Paints and Plastics. It's nearby to Chinook Centre. I don't know their phone number offhand but they do have a webpage I'm sure you could surf through there and find the number for their Calgary franchise.
www.ippnet.com
Another place people mention is GE Polymers and Shapes. They are just off Deerfoot, near either 32nd Av or 64th Av NE or thereabouts.