From what Mom and my sister told me of them, they are sold retail. But what they essentially are is whole wheat, like the kind straight in from the field...in a cloth bag. If you are remotely handy with a sewing machine, you could have one from start to finish in about 20 minutes...
Now, if I tell you how to make them, am I posting in the wrong thread? Ya know like, Women's Views > Non Diving Realated > DIY LOL!!!
Get the wheat from the store. Use the material like what a permenent press shirt is made from. Cut it to the size and shape you want... I have one that is about 8 x 8 finished and one 5 x 18 finished. Sew it on three sides, dump the wheat in, do not fill like a pillow, if anything it should appear under filled. The 5 x 18 one has about 4 cups of rice in it .Yes you can use rice also. You can also use popcorn, but I would make sure it is NOT popping corn, LOL. Sew the last side. Make a washable cover for it with nice soft flannel that slips on like a pillow case. Put in microwave for a minute... enjoy..
[leaving now to reheat mine] Awwwhhh...