Post 10 nailed it, along with the guy who mentioned that anything can be sold out. 'SHTF' don't do well here; but if the Projected Unpleasantness were to occur, you can pretty well forget buying any high-demand item including ammo. Besides, all it takes to ruin your little corner of the US economy is for a car to hit a light pole. Ever been in a checkout line when the power went out?
Get stocked and stay stocked. A couple-hundred rounds of rifle ammo will last several years if you don't waste it into a dirt pile. I've bought a little ammo along, when and where I can find it at or near pre-panic prices. Otherwise, I'll get through this mess just like I did the last one. When you make your own ammo, there are no shortages.