Most are more than willing to give them away. They'll just crush them anyway. Another good source of good heavy duty boxes are liquor stores. Most liquor stores will require you to take the inserts with you. It's not a bad idea because you can use the inserts to pack dishes, glasses or anything else that needs padding.
Office complexes are good for boxes.
I like using the copy paper boxes. They are really heavy duty and stack pretty nicely. You can get large boxes that adult diapers comes in from Nursing homes. You can't stuff them with really heavy items but they are fairly large and good for stereos and such. Home Electronic stores are another good source for heavy duty boxes for your TV's and such.