Offices & retail stores.
Furniture stores are good, too. The boxes are a little large, but they are always double-layer, and often triple-layer, cardboard.
-slym
I once sent three comics in a recliner box. I spent $8,376 on packing peanuts
You joke, but I actually received an X-Men #1 many years ago in a box the size of a small chair!
Kept me in peanuts for months.
I received a Chew #1 CGC 9.8 in a box the size of a small refrigerator.
I refused delivery, and had the postman return it to the post office.
The next day at the post office I had a supervisor watch me open the box.
One slab, a lot of newspaper, and a big azz box.