I agree, ammo boxes are pretty much the ultimate cache containers.
I would check out any surplus stores around Ft Lewis/McChord. They should have them. I was driving by Ed's Surplus in Lynnwood last week and there were hundreds stacked outside the store! (I picked up 2 of the .30-cal size, $5 each). You can find them for as little as $2 if you're lucky.
Good hunting!