Supermarkets are great places to get them. Having worked at a market in a small town around here, we couldn't get rid of them fast enough.
It saves the store the trouble of having to bale them and such.
If a general floor clerk doesn't have any, check with produce, meat, deli, bakery, any particular department should have tons and tons of the buggers.
Also check with your local fast food restaurants, most have a pretty standard size box for their frozen fries, they hold about 40lbs so they're pretty sturdy and the standard size helps with stacking. I recommend going in, don't call ahead, when they're not busy and just ask them. Most will be happy to give you a bunch.
I have also heard that you should find your local starbucks, and ask what day they get their order. They will have more than enough boxes to give away.
Though I have never tried the last one, though I just might later this year.