I seriously doubt if you find one. It MIGHT be possible to modify one made for 308 to work. But I'd not spend that kind of money unless I knew it would work.
I have Winchester 70's in both 308 and 300 WSM. The mag boxes, stocks and bottom metal will interchange. The 300 WSM uses a different follower so if doing a bottom metal swap the correct follower must be used. At least on a Winchester these parts are interchangeable, can't say for sure about Remington.
The only other difference is that the case on the 300 WSM is larger in diameter. This is where I'm not sure, it might be possible to modify the feed lips on the magazine to work with the fatter case. Might not.
The 300 WSM is a niche round with a small following. It is a good round, but I just don't see enough interest for a lot of aftermarket parts specific to the round.
