I actually have bought wood from both big box stores, but mostly pressure treated boards and plywood. If you pick through, you can normally find something straight and not full of knots and defects, but it can take some digging. Almost everything they offer is crazy wet still, so I often sticker it and let it dry a bit in whatever environment it's going to end up in. Hasn't caused too much warping for me yet. I've bought some poplar there, too, which was actually in really good shape, but was way overpriced.

Beyond that, I've had really good experience with John Wilson Lumber in western Howard County, and Freestate Timbers in Timonium. Freestate has a pretty crazy selection of slabs and boards, JW has more sheet goods. Both can cut things for you.

Beyond that, check craigslist and Facebook marketplace. There are often either small mill operations or people getting rid of a fallen tree or downsizing their board collection or selling leftover stuff from projects. Obviously selection can be rough, but you can get some great deals.