There is usually a lot of them out there. It's the specs that are hard to find sometimes. I always looked for at least 102" x 102" and that's a tough one to find.
Here are links to the 2 places I can usually find something. If the moderators have a problem with the links, just delete the post, I understand.
1)
http://www.truckpaper.com/ Once you get to the homepage, you have to look in two seperate places to actually find the van bodies. Click on Heavy Duty trucks and scroll down the next page to near the bottom where it says "Truck Bodies Only", then look for "van", "reefer", "other", etc, depending on what you need. *Important* Go back and do the same thing for Medium Duty trucks. Some post in both sections and some post only in one of the 2 sections, so you need to look at both.
2)
http://www.trucksforsale.com/inventory/22/misc-for-sale These guys used to have several boxes in the EO classifieds and they usually have some boxes for sale. Not sure if they are a paid advertiser or not. Their prices were always pretty high, especially if you have them do the install, etc. But, if your in an emergency situation, might be worth a look.
Added: Ebay is always worth a look too. I have seen some good deals on used boxes on there.