As weird as it sounds, I did a search on Craigslist and found a great box supplier. There are a lot of distribution companies that are willing to sell extra in bulk for extra cash. I bought about $80 worth of boxes for about 1/2 and no shipping. They were located on my husbands route home from work.
I have used ULINE a lot but agree the shipping is a killer. I use boxes that are a lot bigger and the shipping has been $20 - 30 to get my boxes. Ouch!