Hi sneezeallday,
Post #4 here has quite a few sources for any of the materials you may want. There may also be local foam shops that sell foam and most of these would also be able to tell you if their foam in made in North America.
Have you talked to them? As far as I know their memory foam is manufactured in China but their polyfoam is manufactured in North America. Manufacturers can change their sources though and better manufacturers will be open about the materials they use in their mattresses or supply to customers as components so all you have to do is call them and ask 
Phoenix