Gaylord boxes (used)

If you need heavy duty check with the chicken processing plants. They use the thick plastic ones that can be forklifted and are tough enough to take the abuse of tossing in electric motors and such. These are too expensive to buy new, but they damage them from time to time and discard them.

I also use the ones like Patriot has. I get them when I order bulk farm chemicals.

Like others said, watermelon boxes are good and since it's watermelon season right now in our area they should be pretty easy to find.