I think that you need to plan on having or reloading any ammo you will need.
That is one reason why .22LR is such a good choice for some firearm needs. You could literally store hundreds of thousand rounds if you needed. Yes it will not fill all roles but it is a good way to conserve the other more expensive and perhaps rare calibers for more specific applications. You could hunt with the .22 and save your .357, 9MM, .30-06, .270 etc. for other stuff like defense and larger game.
I would not expect to have a chance to buy ammo during a crisis even at the very onset. It is like thinking you will be able to buy gas but there will be lines and not enough to go around. Does anyone remember the last 2-3 months before the end of 1999? I worked at a large REI and we were super busy and kept running out of EVERYTHING.
Whatever it may be; if you don't have it "before" you will not be able to get it "during" or "after" and you probably would be more safe if you didn't.
Stockpile anyway.