What PALady is describing is rice in material. Sometimes they add Flax seed as well. It is a wonderful thing that I found when my MIL got me a store bought one and I ripped it was wondered where all the rice came from!!!
I sent you a link through e-mail to a great product if you don't want to add rice to a sock (weird but it does work )
It is made for womens problems but, due to its shape and make, is great for back pain.