Every sawmill/timber merchant/whatever near me basically sells fences and roof trusses.
For example: Home Depot (which is a national chain of home improvement stores) generally stocks pine, red oak, some yellow poplar (which is not actually a poplar, but I digress), and sometimes something they'll call mahogany (which almost certainly isn't Swietenia anything) . That is about as good as you'll find unless you go somewhere that specializes in hardwood lumber. Unfortunately, some of those don't do retail sales.
For US readers in the northeast, Highland Hardwoods and Northland Forest Products both do retail sales. Northland is more focused on North American wood, though they stock FSC certified mahogany. They're about 10 minutes from each other about an hour and 15 minutes from Boston (usual traffic caveats apply), where I suspect a lot of people from HN at this time are located.