I think it depends what you are wanting to spend.
I bought a Kilbros gravity wagon with loose gear, but 3 near new tires for $750. Spent a cuple short afternoons fixing it up, and now it tracks fairly well, with less than $50 worth of parts put into it, plus it needed the one old tire replaced during harvest.
The box itself was in very straight condition., just a few little cracks here and there.
Maybe check some place like Bigiron.com for one in your neighborhood? If you have to pull one with wobbly gear any kind of distance, take some 5/16 chain, and a couple load binders, and chain the tongue straight forward. It will 'skid' a bit around corners, but you'll be able to drive it more than 15 MPH without it wobbling all over the road.
If you want a new box, I'm sure dealers can order one in.
