Hit up Walmart or any large store like that. When I worked there people would usually come in asking for any boxes and stuff and we would load them up with as many as they needed since all we do with them is make them into small cardboard bricks. I would suggest going in early as thats when most of the stuff is unpacked.
Talk to your company's IT depatment; or a friend who works for a company that has a large IT department.
The boxes that computers come in are great. Good, solid, and strong. Bigger boxes also often have handles.
That's a great idea. I just finished moving and I re-used the same two computer boxes I found in my dad's garage about 10 times. I'm going to have to remember that for the next time I move.