We make and have lots of patterns for bags here in TQAville. There are many companies making all kinds of interfacings, and sometimes it's hard to figure out what type of interfacing is being called for in the pattern requirements.
Sara over at Sew Sweetness just did a fabulous blog post with everything you'd ever want to know about Pellon interfaces for bags. We carry most of them here in the shop.
Photo from Sara's Sew Sweetness blog
Check it out! You might learn something new! I know I did.....