Good luck. Every time I've seen somebody with used boxes they seem to want about as much as new ones, which are expensive as you know.
I don't know what kind of project you have going there, but for that many boxes have you considered building them your self all in one unit exactly the size you need? Compartment doors are not that expensive to have built to your order already framed with hinges and latches, and if you get some at the surplus in about the size you need they are downright cheap. I paid $15 each for my rv compartment doors at Bontrager's, but you really need to be able to go there and pick through the surplus stuff.