I dropped into the Uhaul store to buy a couple of boxes for a project and they pointed me at a stack of free used ones.
Don't know if all Uhaul stores do this. This was at Boulder hwy across from Eastside Cannery by Denny's.
If you work at or have a hook up at one of the local hospitals their supply department does. I was able to get dozens of great shipping boxes so I could move.
When we moved here to Sandy Valley, we used a lot of IKEA blue bags. you would be surprised how much you can get in them, I have about 300 tune up manuals and I got them all in about 10 bags, they weighed about 250-300lbs all together.