There's the Freighliner dealer, of course.
First call I'd make is to Automotive Specialists of Broken Arrow, since I know they work on Sprinters. 918-251-1067
Another one to call is VAP Auto Shop at (918) 398-6670. I've never had anything fixed there, but I delivered there once. They're a large shop that specializes in Mercedes and European vehicles, but they work on most everything.
Failing that, I'd give Silverstar Transmission in Edmund (OK City) a call at 405-330-9300 and ask them for a recommendation. They'll know.