Check the Yellow Pages under "Plastic Suppliers": there should be some in Sudbury (I had to check a map to find out where Coniston is
ops: ) You can buy a 4'x8' sheet for less than $30.00. The first sheet that I bought was .040", but I find the .060" is much more useful, especially for larger structures. It's thick enough that you can edge-glue narrow strips to the interior of structures as bracing, which is much cheaper than buying .100" or .125" strip for the same purpose. I use lacquer thinner as a cheap and very effective solvent-type cement. And don't let the size of the sheet put you off, as far as transporting it goes: the dealer will roll it up and tape it, so that it'll fit in the trunk or backseat of a car.
Wayne