i don't know about RM for shoulder straps, but there is an eBay seller that sells replacement parts for bags, and their shoulder straps are really nicely made, if you search i am sure there was a thread with their info. mind you they only come in standard colours ie. if you are looking at a strap for bright blue mam you may be out of luck, or maybe you could go to a leatherworker and have one made?
don't know if this helps, but good luck!