Best Sources for Packing/Moving Boxes?

It seems silly to purchases boxes when so many can be reused and so many others are being thrown out.

Upon hitting up stores recently that often give their extras away from the warehouse, one friend found that many are now just saying "NO."

Then, on a recent trek to the always ghastly busy and inconvenient U-Haul on Fullerton Ave. to find boxes, my friend found a pleasant surprise: there's a spot for drop-off (donate) or grab (borrow)...

With so many people we know packing and moving right now, what are your best resources for boxes (with no purchase required)??

Related:
The Best and Worst of Packing Up a Home

(Image: Alders Removal and Storage)