If you have a sewing machine, you can make your own! You don't have to be wonderfully crafty. Just buy a small bit of muslin or other 100% cotton material and some flannel. Make a bag the size you'd like out of the muslin and fill it 3/4 full with dry, uncooked rice or flax seed. Then make a slightly larger cover out of the flannel.
You can add dried herbs directly into the bag or put a couple of drops of your favorite essential oil on a cotton ball and place inside the cover.
The muslin bag of rice or seed can be microwaved. But don't microwave essential oils - add them after the bag is warmed.
If you're not interested in making your own, here's a nice website with a large variety of styles... http://www.aromatherapyricebags.com/