I ran across a tutorial the other day for a bed warmer. This is some info included with the tute regarding filler:
You can make a bed warmer filled with feed corn, rice or buckwheat. I use feed corn, because it is very inexpensive and it stays warm for an exceptionally long time. I purchase feed corn from our local IFA, a feed and farm supply store. A 50 pound bag is less than $10. That’s enough feed corn to make between 17 to 25 bed warmers. You can also purchase it by the pound if you are only making a few, but it is not as cost effective. Buckwheat (use whole seed, not the hulls) can be purchased in bulk from many natural foods stores. Buckwheat is wonderful, but just more expensive. Rice is inexpensive, but doesn’t hold the heat as long. Be prepared for what ever you use to smell when it’s heated- feed corn like popcorn, rice like cooked rice and buckwheat… smells a little grainy, but has the least smell of the three.
We have found several fabrics that make good warmers, such as cotton chenille, cotton flannel or cotton fabric. I prefer chenille or flannel as you want it to be cozy and soft. The only thing we’ve found is that the fabric should be a natural fiber, for some reason the synthetic fabrics seem to sweat when the bed warmer is heated (we found out the hard way using minky.)