In my area, we have some grocery stores that have bulk foods. If not there check out health food stores. BUT if you can not find any, rice (regular not instant) can be used in its place. I am assuming that this is for a "heating pad"?
Flax seed is more expensive than rice. When heated flax seed does not (I am told) have a oder, rice does, not a bad one (in fact DGD loves the smell of my rice filled heating pad), just a little different.