Check for a local plastics supplier near you. Often they have irregular sized pieces for sale at bargain prices that were cut from larger pieces.
btw, polycarbonate and acrylic aren't really that great for exterior use. Acrylic tends to yellow from ultraviolet light exposure and polycarbonate scratches fairly easily. Polycarb will cloud up in a year or two just from the wind (and the dust particles contained in it) blowing against it. You'd be better off having a piece of shatter-resistant glass cut if it's going to be used as a window.