I use shredded paper. Buy a paper shredder! Use all that paper for packing instead of throwing it away. Works Great!!!
Take it from a man who has moved more than a dozen times, the best place to get great boxes for free is from grocery stores at 3am!
Depending on what you are shipping, the key is to double box the blasted thing! Always double box and your buyer will love you for it! Secondly, make sure that the inner space is about 4" on all sides as well as top and bottom. Then get a sheet of 4' x 8' x 1" styrofoam insulation from Home Depot (or the like) for a couple of dollars. Break off pieces of the foam to fill all of the 4" interior spaces, and make sure to stuff it tightly so that the foam presses against both your inner (product) box and your outer (grocery store) box. If it's packed too loose, the foam will be of little protective use. Stay away from the packing peanuts (there was a thread on this yesterday) because they are useless for heavy items like amps.
If you are shipping large speakers and don't have the original boxes, then shame on you! (It really helps for warrenty purposes to send items back in their original packing materials). For all other items, including small speakers and anything that would fit in a standard rack space, the double boxing with foam works every time.
In the Yellow Pages under Packing Materials you will find the best deals for supplies-as compared to the places you mentioned. Unless otherwise stated they all have city counters & will sell to you. The downside is sometimes the minimum purchase is a large quantity but not always. The smallest bag of packing peanuts at the place I use is 14 cu. ft. To go off on a tangent briefly I have to say that I've used lots of packing peanuts as the filler when double boxing & have never had a complaint, let alone a problem. When used correctly, just like anything, they do what they're designed for.
I have a never ending supply of boxes and in fact I drove by a store today that sells ceramics, lights & such & there was a pick up truck load of boxes just sitting there. My "shipping dept" (attic) is loaded so I didn't bother stopping but just start asking around-maybe where you work if they have a receiving dept. or mailroom or anyplace you can think of that may be willing to part with some boxes, like an auto body shop. It's doubtful they'll be sending that fender back to Detroit.
One more thing to add. I've been in a lot of grocery warehouses & I would never bring a grocery store box into my house. I hope everybody reading this really does wash the tops of their cans off before using them, especially soda.
Sears is good for large boxes, my local Sears outlet will let me come down in the morning 1st thing and pick out what I want. Places that sell large office copiers usually have great boxes as well. Foam 4x8 1/2 or 3/4 thick sheets are available from Home Depot or the like. Cut easy with a razor knife, and with tape you can make about any kind of foam support you need.
One day come with the car or truck to PC Richard's, Toys'R'Us or any major store and ask for boxes. You're not going to imagine how many you can get of a different sizes.
Besides, you can tear some of them to have extra carton for dividing components or fortifying the box. So the only probably thing that left for you to buy is the bubblewrap that you can get cheaper at the truck rentals office or Home Depot rather than in Staple's.