Welcome to the forum. The Holster Store (check their web site) seems to have one that might be satisfactory. They call it the Pro-Carry Belt Ride. They say it is for two-inch guns, but because it is an open-toe design it would accommodate the slightly longer J-magnum barrels. Not sure about the additional cylinder length, though.
I would think any J-magnum holster would work, though the physical profile of the 649 might make it appear to fit oddly. You might be in custom territory if you are looking for way to hold a 649 on your belt that is both secure and good-looking.
I regard the 649 highly but mostly think of it as a pocket carry firearm.