I have used buckwheat for neck wrap filler, and it works great. It is microwavable, but if you leave it in too long it burns and smells not good! I haven't bought any since I lived in Colorado, but got it at a feed store.
We had a mouse visit us a few weeks ago, and he loved the buckwheat neck wrap. Needless to say, I live-trapped the little sucker and drove him about five miles away before letting him go (and threw out the neck wrap!).