Man I used to work for Lowe's back in the day, and they're always practically GIVING stuff away, you just have to ask for it. There's always some busted plywood and stuff coming in, or just something small wrong with it where they can't sell the whole sheet for full price. If you catch them at the right time you can make out like a bandit, perfect for this kind of stuff.
If you got one near you I'd make an appearance and tell them what you're doing and give them your name and number, they'll give you a call if you're persistent. And if you come to take the stuff off their hands when its needed more than once or twice, you'll definitely be the first person they call whenever they get a "bad" batch.
*Edit =
Actually Skidude there's still some people working there at the Millington Lowe's that I know, maybe even a few still in the receiving department, which means I could get first dibs. Kind of give me an idea as to what you're looking for and I'll tell them to keep an eye out. Or you know... accidentally drop a load...it happens.
And if you want pallets, I know I can get those, if anybody has a trailer we could load it up and haul it down there one day.