If you're looking for a great, affordable source of plastic surfaces to paint, check out this selection on Amazon. These products are all highly rated and many even come with free shipping.
You could also check out your local hardware store or art supply shop. Many hardware stores like Home Depot sell garden supplies – so look for items for sale in their gardening section.
Or, find an art store in your area that carries items like blank canvases or sold paintings; these will typically have nice wooden frames that can be repurposed into new pieces.