Hi, about your purse, I use "Tim Tex", it's a very stiff material that looks like a piece of batting surface but very stiff. Can be cut very easy with Offa cutter. Most of the time I just order from on line stores. It comes by the yard an is 18" wide and costs about 5 or 6 dollars per yd. if I remember right. ( google it and price it out at different shops). Measure the bottom of your purse pattern when sewing together and cut to that measurement. I also cover it with lining fabric an sew in a grid back & forth over the top for more stiffness. Usually about 6" wide on most bags I've made. Don't bend it, and that's easy to do. When I make bags I do cut a piece of lt. weight batting the same size as the sides and use it like 1 piece, gives body to the sides. I use warm & natural batting for this. I don't use the fluffy stuff in the sides.
good luck
Shirley/Tn.