Plexiglass, or sheet acrylic, is NOT very expensive, unless you are buying it at a sewing store :D
Any hardware store, home improvement store or the like will cut to size for you. Here is a link to an example available at Home Depot, it's 18 x 24 and for about $7:
http://www.homedepot.com/webapp/wcs/...02&R=100402508
Just tell them you need to have it cut carefully SQUARE, in whatever size you need.