Custom is gonna be the only way to go, as for wait time......well you may not have an option. The traditional muzzleloading barrel makers likely do not start out with that large of material for making their barrels, so you might need to check with barrel makers of the modern kind, especially those that supply military. I know Badger Barrel in Bristol, Wi. Would make you anything, you might have to drill and thread your breach end (or pay and give specs to what you want there). They can drill a hole through any dia. Material and just leave in the the rough. I've seen how large diameter they start out with to make 50cal machine gun barrels since they machine the compensators, they are not the screw on type, they are permanent. But, you will wait, these guys don't stop production orders for a one off barrel order. You might get lucky and they have something that could work for you too. Also remember, only a large lathe will be able to handle such a barrel, a lot can not pass a 2" barrel thru headstock, think about that if you have to breech such a barrel. Good luck on your search.