|Hardware stores carry a 2'X4' plastic panel or sheet that fits into a ceiling grid covering flourescent lights. They come in various designs and are flexible yet firm in small pieces. I used one cut to fit in the bottom of the purse I made so that the purse wouldn't collapse when I sit it down.
My original intent was to cover it in with my lining fabric, but since it is transparent, I left it as is. This seems to be working well and has much more body than the plastic needlepoint sheets I've seen recommended.
Cutting the material is a little tricky--my husband used a Dremel wheel. The best part is that you could get LOTS of purse reinforcements for very little money.