C.G.
We have moved A LOT and I have to say it is well worth it to buy the boxes you need in the proper size. We just moved and bought ours at Home Depot. We probably spent about $100 total on boxes (for around 30 boxes I think). They are around $2 a piece depending on size. In the past we have begged boxes and used what we got and it was MUCH harder than just conveniently having the size box we needed. Also because they were new we saved them in our basement and will use them again. Good luck with your move.
By the way if you do decided to buy them from Home Depot don't order them online. The delivery is very expensive but they actually have the boxes in the stores (the website says they don't!) so you can just go grab them. We got ours at the Home Depot on Schaumburg Rd/Barrington.