For standard as well as more exotic shapes
McMaster is good, but you pay a premium just like most of their stuff, especially on shipping of odd dimentioned products like sheets. To avoid shipping costs most hardware stores (Lowes, Home Depot, etc...) have a basic selection, usually by the storm windows.
Also, just in case, a word of warning, acrylic and polycarbonate are not even close to the same. I can't even think of an application of acrylic on an FRC robot besides one designed to generate shrapnel.
