Grocery store, or a school cafeteria or assisted living facility. Call and ask when they get their deliveries and they are more than happy to give them away.
We throw away a lot of heavy boxes at my work each week. Most of my family has used them to move one time or another.
If you get down to the Lake Village area, I can bring them home any night. I will be up in Porter County this Friday evening by I94 and Rt 49 and/or plan on shooting at Winamac on the 30th.
If you want to buy them go to a unit storage place, most of them sell boxes as well. If you want free boxes, just got to any store and ask them if they have some to give away. My wife will go to stores that sell large appliances and get large boxes for her students to do things with.
I can get you wine and liquor boxes from work if you want. I don't know how many you need, but just let me know, I will put them in my car instead of crushing them. Liquor stores will usually save them for you as well.