Of all the firearms manufacturing processes, casting bullets is one of the easier ones. There will always be lead or some suitable substitute available one way or another.
Most lead ore mined in the US was exported for smelting before this. And lead is the #1 recycled metal. (Think car batteries). One old car battery probably contains enough lead for a lifetime supply of .22LR bullets.
There is plenty of lead ore around, plenty of countries willing to do the smelting, and plenty of old batteries. So I doubt I'll run out of ammo before they throw dirt on top of me.