When I was in Wall-Mart yesterday, I noticed that the ammo shelves were almost empty! I am glad I stocked up last year.
After the looting in New Orleans Walmart started keeping many of their weapons and ammo in the back, under lock and key. If you ask the salesperson for a particular type they might have it out back.
Order it online.
This is the place that everyone I know seems to have the best luck with.
http://www.ammunitiontogo.com/catalog1/index.php