purse help

[cellarstella.livejournal.com]

I checked the memories section and found nothing that was specific enough... I want to make a purse that has stiff sides, and a stiff middle divider because I want it to have seperate sections that will hold up well and keep a nice shape. I'm not sure what to use for the stiff pieces though... stencil plastic? any other/better plastic? I don't want to use cardboard because it can get soggy in my rainy city. Is there something specific I can find at a fabric store?