I get my packaging supplies from clearbags.com. I've been very happy with the shipping time and quality.
I work at a print shop and have a greeting card company. I get my supplies from the same place we buy paper. The larger paper companies carry boxes of different sizes and other packaging supplies. I'm not sure of what companies you have up north, but you could check with your local printer to see where they get supplies. You might need an account or a minimum order, unless you're able to pick up yourself. Just a thought. Good luck!