This is what I use....the purses are plenty firm.

Lazy Girl Designs makes an acrylic bag bottom. There are several sizes to choose from. The bottom is covered with a slip-on sleeve. I have made several purses and interchange the bottom from purse to purse. If you don't want to buy the bottoms. You could buy the acrylic plastic sheet and make your own.
To make the purse stronger on the sides. You can layer between your inner and outer fabric a product called Steam-A-Seam or maybe quilt the layers.
http://www.lazygirldesigns.com/produ..._E_Bottoms/c53

Polastic canvas sounds like a good idea. I have cut up old plastic corrugated political yard signs for the bottom. Cut to size and cover with fabric.

I would like to use the Soft and Stable, but it is pricey. I have used and will probably continue to use medium weight Pellon fusible interfacing. It is plenty firm, but not stiff. I generally add a piece to my inner pockets, too. I buy the Pellon at my local Joann's with a coupon.....is really inexpensive that way.....:)

I have used that canvas and also a piece of foam core board cut to fit and covered with a pillowcase. And I found the bag bottom at a quilt store mentioned above that I kept for myself!! It can go in a pillowcase and I can move it to different bags.

Sewing with Nancy just had a couple of shows on bags and you can view these on her website free www.sewingwithnancy.com and she shows some alternatives for the bottoms and for the stabilizers. Good shows.

20+ years ago, I made a diaper bag for a friend who was having a baby. I used plastic canvas for the sides and bottom. It was substantial and washable.

I'm with you. I like my bags with good body. Here's what I do --

I use fusible fleece instead of batting. This, by itself, will give the main bag some body.
But to add a firmer feel (so it stands up by itself), I use a medium fusible interfacing on the lining. Don't use heavyweight, it will be too hard to turn through the lining hole if your pattern uses the "birthing" method.
For the bottom, I use plastic canvas.

These tips have come from trial and error over many bags (sell them locally and online and no complaints).

Hope this helps!

What do you cut the with?

the curve makes the plastic canvas rigid. without the curve, plastic canvas bends

I've made a few bags and Iused mat board for the bottom
maybe you can get off cuts from a framing shop .
I hpoe this will help.
Noella.