I usually keep different types of interfacing on hand at all times. I am picky about what I use - I prefer fusible horsehair (ACRO) in totebags because it's woven and strong. I use fusible fleece in little bags because it adds shape. And I use a lightweight fusible in lots of projects just to give the fabric some strength.
A few months ago I learned about a new type of stabilizer called Soft and Stable by Annie. It was a thin foam-based stabilizer. I loved to work with it because it added shape without adding bulk. And it was a breeze to sew through (unlike fusible fleece). The only drawback was the cost: $11.95 for an 18" x 58" package. Yikes.
So I started to explore some alternatives - I knew I had seen something similar to this foam stuff at JoAnn Fabrics. After some research online, I found it: 1/4" scrimback sew foam. It's not quite the same, but (60" wide) it's considerably cheaper...so who cares!
I decided to head to JoAnn to see if I could find it. And I did. It's over in the upholstery fabric section. In the store near me, it comes in tan and gray. And just like everyone else, I used my coupon (and Teacher's Rewards) and got it for a lot less than $14.99 a yard.
I've used this as a sew-in stabilizer for several projects. I love how it gives the bag considerable shape without adding bulk. I don't know how long it will last, but this stuff is designed for car upholstery!
Here is a tote bag I just finished for a friend of mine. It is standing on it's own - I'm just holding the straps up!
And here's a diaper bag I made using the Soft and Stable. I love the shape that the stabilizer gives the bag. Awesome!
I'm not sure if I would use it for all bags, but it's worth a try! The only directions for using this is to place the foam against the fabric and the scrim to the outside. The scrim prevents your stitches from tearing the foam.
UPDATE! I've got a treat for you. Here's a free pattern for a round zippered pouch...but you don't have to cut anything round! Yippee! I was inspired by this free heart-shaped machine embroidery design.