good question, I would definitely look on Uline and mailersUSA, they have some good boxes and things for heavy stuff, and there are a billion sizes of mailers or boxes for large items, although they do of course get more expensive, and if you buy in bulk they take up soo much room!
do you ship heavy through through the post office or from home? you could always do heavy through the post office flat rate, that might help out. all of my items so far have been super light so I've got no experience in that area!