I would use a interfacing, whether a mid-weight iron-on or like Deco-bond. I used the prescribed Deco-bond in a "Professional tote" that I made and I would be very unhappy if I would not of added the interfacing, It would of been awful flimzy. I have a pattern to do this little wallet and my plans are to add the interfacing.
I only used Heat and Bond and Wonder-under for applique. Course now there is Steam-a-seam II, that is what I use for my applique work. Heat and Bond messes up my machine and needles.

you can use any fabric for an interfacing; muslin works well. and as for the wonder under or another fusable...you can use a fusable in your bag but it has to be ironed to something; interfacing is a fabric used to add a layer, giving body to your piece, fusable does not add that layer, it simply works like glue to hold pieces. 2 very different animals. but, any lightweight inexpensive fabric can be used instead of purchasing a separate interfacing.

I'd just sew it in there if your heat/bond is heavy. It can cause needle problems. But if realy light, go for it. Its only fabric and thread! :) I love finding stuff at home. Its a little challenge for me. We live in the boonies so kind of hard to run out for just one thing.
Let us know what you do and how it works! :)

Thanks to everyone for the tips. :)

I'm going to experiment (possibly with the jeans idea) and see what happens. I'll let you all know how it works out. :)

Cool, I'm interested in how it comes out using jeans as an interfacing. It will definitely be sturdy, which is what interfacing will do. You might have to quilt it to keep it together.

I have made a ton of bags/purses and always use interfacing in them. Someone above said they would flimsy without and she's right.

Just wanted to post a link here to my semi-finished wallet. The jean worked great! If anything it was a bit TOO stiff, but I think it will work out alright. :)

http://www.quiltingboard.com/t-68454-1.htm#1679989

I would not make it without pellon. I have used batting but I didn't like it as well.

Don't worry, after being used it will break in quite nicely. Great Job!