Nothing in our rations can be purchased in supermarkets.
None of it occurs in nature!
None!
Winco used to sell MRE components, but stopped before my 40th birthday. I celebrated with pizza in the park, but I wanted to buy a bunch of magnesium powder heater packs, pour the contents into a simple envelope, hide it under briquettes, and then act surprised [from a safe distance] when it flares up.
Based on my experience it would flare way up.