If you are in a hurry, you can simply to to any hardware store and buy moving boxes. Home depot, lowes or any hardware store will have moving boxes. BUT you will pay a premium for boxes. They range in price from 50 cents to $10/box.
If you have time to do some bargain shopping, you can do an internet search for used boxes. Simple type in Used or discount moving boxes under a local search and you will find some retailers. Another good resource is your local craigslist. Many times people sell their left overs. I sold over 100 boxes from our last move for $25 in Sept. 2010 to a very happy craigslist buyer. (it would have cost her over $300 buying new).You can often make a free score by driving a new build neigborhood on trash day and find some in the piles by the curb.
How many boxes do you need for your move. The average 1500 square foot home will need about 60-75. NOT including garage or buildings. A two car garage will need 40-50. Our 2500 square foot home, garage and storage shed took 200 boxes, give or take.