RE: Does this scope exist?
There are a lot of rifle scopes with those specifications. They will work just fine on most muzzleloaders. I have a Knight with a 3-9x40mm Sightron SI, another Knight with a 3-9x40mm Millet Gold Buck, and both of them are excellent rifle scopes although they are not "muzzleloader/shotgun" scopes.
Just keep an eye on the eye relief. A real good one that people seem to like is the Bushnell Elite 3200 which is a 3-9x40mm.