I'm not aware of any commercial flintlock blunderbus produced at the present time.
Any number of custom makers can make one for you. The question is how historically accurate do you want it and here you have to find one who is knowledgable about them who is willing to take on that project. Ron Ehlert comes to mind as does the folks at The Rifle Shoppe (BTW, I just ordered the parts for a grenade launcher from them).
Then again, if you want to spend the time and am reasonably skilled with basic tools (chisels, file, scrapers, spokeshave, planes), you can make one yourself! Vernon Davis (6901 Running Deer PL, Dublin, Ohion 43017, ph: 1-614-761-2568) makes 20" barrels prefitted with tang in 4 or 10 ga, steel for $200 and brass for $290. It's an octagon affair near the tang and later becomes round. BTW, if you can wait for the National Matches (blackpowder that is) at Friendship, IN, you can probably get the barrels cheaper.