What happens when the other country isn't willing to sell? Consider a war or other disaster e.g. in the months before the pandemic, China stopped the export of face masks and other PPE, resulting in shortages in hospitals around the world.

There at other manufacturing centers than China. I don't know why people keep it repeating this line. Where do you think China gets their things made? It SE Asian countries. And they're making in roads to Africa. That's what we should be doing. We have a president talking about shhole countries-- destroying international relations is gonna do a lot more damage in a war. We should be building diverse business relationships with competing economies.

Countries are virtually never 'unwilling to sell', and there are quite a few different countries out there.

PPE shortages in hospitals were because of unprecedented demand levels, not because China didn't continue to export it. A country is actually a lot more likely to have problems handling that kind of short term demand shock if it relies entirely on domestic manufacturing

Free market - money decide.

If not - the politics is involved in selling decision making - then the US starts a sanctions war, as China example shows. It sells in the end, but after being economically punished.