In Seattle, we picked up billboard vinyl for free from Clear Channel. They do provide some of their vinyl for some local artist groups and such, but they still have more. We found their local head's contact info by just putting in a request on their website. Once we got the contact info, we just called the guy up and went to pick up a couple huge billboards the next day. It might be worth seeing if you can get local contact info for their warehouse nearest you or the next couple over, through this method.
If no viable free source comes up, this place: http://www.pointonepremiums.com/
has a variety of sizes and types of used billboard vinyl for sale - pretty reasonable compared to a lot of other covering options, still.
Good luck : - )