Amy Butler wrote an interfacing guide for us awhile back and recommended Stacy's Stabilizer 101 layered with Thermolam Fleece--both Pellon products. I've been using both since and love the weight and sturdiness of the combination. We don't carry any interfacing, but Jo-Ann online, or Nancy's Notions might have them.
http://sewmamasew.com/blog2/?p=154