Where can I find heat packs for plant shipping?

Kinda off the topic but I need some help or info?

I am going to ship off to a freind some water lettuce. Its freezing here in PA and its going to oregon.... I want to place a heat pack in the box with the plants to assure they make it ok... Can anyone tell me besides online where I could buy these? Does any other stores sell these heat packs at all?

Thanks for the help and info?

Spankey