I bought some plastic shelves at walmart for about 3 bucks each... They are made of a PVC like material, and while they seem a bit flimsy, they hold my 11x14's without problems. I haven't stacked them, although they are stackable, I simply use them on my work table, one tray on the shelve, one bellow it.

The shelves are like those "space savers" you stack inside closets... I'm not sure how else to describe them, but they are fairly cheap....