The odd-ballness of the 5.7x28 is a double edged sword. On one hand, it is not to be found at walmart or other narrowly stocked stores.
On the other hand, its a popular caliber, owned by relatively few, well-to-do shooters. That means that now, in the middle of the "GREAT PANIC" that has been sweeping the nation, 5.7x28 is one of the most available calibers on the internet by a wide margin. The price for the SS197 40gr ballistic tip round has remained a pretty constant $.36-$.41 a round, or about $20 for 50 rounds- less than half the price of .223 durring peak gouging. Ammunition to go, Midway, ect have all had 5.7 in stock this whole time, while 9mm, 45, .223 all sells out day after day.
So, in short, it may be hard to find locally, but it is easy to come by online. Just buy a case of 2000 when you get your pistol and you'll be set for quite a while.