There's a bunch of chinese DRAM companies currently playing catchup to get closer to modern densities. Could Apple buy one of those? I'm guessing there would be regulatory hurdles to that on both sides of the pond.

Apple never bought a manufacturer, or built such capabilities.

They buy and build manufacturing capacity, and there’s also a huge shortage in that today.

Which is funny, since until relatively recently DRAM was what you produced in fabs with processes that weren't competitive enough for CPUs anymore.