I would not worry. There are so many 5.45 rifles around that it would take a serious market disruption to dry it up. Its Russia's (and many of her allies) standard-issue round right now.
Most ammo is Russian imports. But if that dried up, I'm sure some domestic company would pick it up.
Its in the better-stocked gun stores around here. Always at gun shows. And, as mentioned earlier, its no trouble buying it by the case online.