You either store a large surplus of them in warehouses, or ensure you can rapidly manufacture them even if the world around you goes to hell. That should be the responsibility of governments, and it's one of the main reasons to even have one - a government doesn't have to justify everything in terms of costs, it can evaluate some non-profit metrics like, I don't know, saving lives in case of emergency. Alas, something happened after the Cold War, all western nations went collectively insane, and got rid of most emergency surpluses and any kind of buffer in the system.