I've been using Pellon, bought from the bolt at JoAnns. I just bought "Stained Glass Quilts" and they recommend Steam A Seam 2. It's available at JoAnns online, I don't know if it is in the store.
I didn't care much for the Pellon, had a heck of a time trying to get the backing off.
Have you used either? Is there a better product?
btw the book "Stained Glass Quilts" sure went down in price. You can buy it at half.com or Amazon for $1.80