There are several good threads in the What type of tree do you use? section:
http://smokinitforums.com/index.php?board=6.0
In addition to the wood you can buy directly from Smokin-It, Maine Grilling Woods and Fruitawood seem to be the favorites around here:
Maine Grilling Woods - http://smokinitforums.com/index.php?topic=2108.0
Fruitawood - http://smokinitforums.com/index.php?topic=1935.0
Big Box store woods are generally kiln dried, which causes excessive combustion/belch issues. So, I steer away from them. But, I know we have a Midwest based hardware store chain (Scheels Hardware) that has pretty good wood. So, Ace could have some good stuff as Dave has mentioned. I would just stay away from the stuff made by Weber and Charbroil as I have not had the best luck with their wood.