You can get them for free at CMP.
Of course, then you'd have to do something with the ammo that comes in them.
I'm not sure what you mean by "properly" storing your ammunition. Anything from emptied peanut butter jars to 5-gallon buckets do a fine job of containing ammunition.
The stuff doesn't have a very good angle of repose. Otherwise, I might just pile it up in a corner...