I have used the buckram that is used for drapery making, and that works well if your bag bottom is not very wide.. I also have used the plastic place mats, and like another quilter wrote, they are usually in expensive, and you can get several from one mat. I make a sleeve, and hand stitch the end,,,,, sure beats the expensive purse bottoms.
also, I shop goodwilll shops for purses that have nice metal holdings for the straps, then I cut the original strap off, and use them on the straps of the purses I make. sure beats the price of purse handles in the LQS... also, sometimes I use the entire leather handles , as they are many times in very good shape too.
I love making purses,,,,and have more patterns than I'll ever use....must be a weakness on my part.
Jacie