I personally have my own WoodMizer sawmill, so I saw my own timbers. There are still a couple of mills in my area that can provide timbers, but they are spread out. You need to buy timbers that meet your criteria. If you find a mill that regularly supplies timber framers, you may be all set. If not, there will have to be an educational period for your mill.

I agree with TIMBEAL about wranglerstar, not always the best info. I recommend Timber Frame Construction, and Build a Classic Timber Framed House, both by Jack Sobon. The first book has plans for a 12'x16' garden shed. Google "sobon shed" for photos. The second book has an 18'x36' hall and parlor colonial.