Good lord, son, you're in Pennsylvania! There are hardwood trees everywhere!
As many others have noted, check craigslist, ads in the paper, look for signs by the road, random trucks full of wood parked by the road, etc.
Do your best to learn how to identify species of wood. It takes time and practice. Find a good reference book that shows pictures of bark and leaves. It helps if you have a friend who knows wood and can go along with you to shop. If you have the means, cut your own wood. There's no better way to learn how to ID wood and get a feel for what good hardwood looks and feels like.
The good news is that the vast majority of hardwoods in PA are at least passably good for smoking. Stay away from anything that is light and pithy, like sycamore, aspen, tulip tree, basswood, and tree of heaven. Black locust is pretty much the only dense hardwood that would be bad for cooking.
When you're looking at a stack of wood that someone else cut and split, it is difficult to tell if it is seasoned. Checking cracks on the ends of the wood tells you that it has at least started to dry out, but they can be misleading. I've seen checking cracks form on wood that has only been cut and split for a few weeks. Smitty suggested a moisture meter. I've never used one because I cut my own wood and I always know how long it has really been seasoning, but if you're buying wood, I think a meter would come in quite handy.
