Where can you buy heat packs?

Winter is coming! And I need to get my little cactus & succulent garden tucked away for the winter so they don't freeze to death. I'd bring them inside, but there would be cat vs. plant issues. I've got my green house set up, I just want to be able to toss heat packs in on the really cold days/nights. I know you can buy them for mailing things in the post for things that are temperature sensitive, I just don't know where to buy them. Thanks!