When I asked this question here a few months ago, someone was kind enough to mention that Monday evening is when the businesses in Davis Square put out their recycling for Tuesday morning pickup. You can find a lot of boxes sitting on the sidewalk on Monday evening (and not just in front of the places that sell liquor).

I also had luck with freecycle and the "free" section of Craigslist.

Also, keep in mind that UHaul will buy back any boxes that you buy but don't end up using. I took advantage of that when it turned out that the boxes I had bought from them were too big.