"...not a combo gun at that point..." Neither is an O/U shotgun.
In the old days, NEF/H&R single shot Handy Rifles could be fitted with shotgun barrels. At extra cost and only in the factory. However, like many things these days, it'll depend on how much money you want to spend. There's no reason a Savage 24 or even a SxS shotgun couldn't be fitted with any barrel you want. Keeping in mind that double rifle barrels by themselves are scarce and really expensive. The technology of regulating(making 'em shoot to the same POI) is an art.
There are lots of drillings available on the assorted auction sites. Nearly all of 'em are made in Germany. Blaser USA, Inc. makes 'em too. Some of 'em run under a grand US, but most run around 3 grand.