I'm making a purse and the pattern calls for a cardboard bottom that just sits inside the purse. This doesn't seem too sturdy to me, any other ideas?
Yes, I think you can use a piece of plexiglass. You can buy it in different thicknesses and if you have those big box stores (like Home Depot or Lowes) nearby you can find it there. To cut it you will need a special cutter but they sell those too. If you can only find thin stuff you can always make a cardboard bottom and then put a piece of plexi on both sides for reinforcement.
If your pattern doesn't have a cover for the bottom you can make one. Just trace around the 'cardboard' bottom, and add a seam allowance. Sew three of the sides up, slip the bottom inside it, then slip-stitch closed.
Hope this helps,