I haven't used them for this, but New England Custom Guns has a great reputation for excellent work and has what looks like one of the better selections of quality swivels and mounts suited for side-by-sides.
https://www.newenglandcustomgun.com/Gun_Services/search.asp?CI=296&ST=Slings%20&%20Swivels
I'm thinking of adding swivels to a couple guns and would probably have NECG do the work.