Lol MREs at supermarkets, that's silly. I'm pretty sure I've got a better idea of what they're like than you do Darth Monkey.
The packaging on MREs claim that they can't be sold anywhere legally, however there aren't any actual laws restricting its resale. In fact if you'll do an Ebay search I'm sure you'll find at least a hundred listings. Also, from my experience it's common for people in the military to give away anything they don't have to turn back in to friends/family so MREs are practically everywhere.
They are cooked by a chemical reaction, not quite an explosion. The reaction causes water to boil.
Chicken teriyaki isn't on the current MRE menu and as far as I'm aware no MRE as far back as 1981 has been available in chicken teriyaki. Are you sure you don't mean beef teriyaki?

I also have older C rations and MCIs but those sit on my collection shelf.