We were on the Rick Steves Adriatic Tour in September and had Alma Elezovic as our local guide in Mostar. We found her to be excellent. She can also be seen on the tv show Rick does of the area. Her contacts as listed in the RS Eastern Europe guide book are; tel. 036/550-514, mobile 061-467-699, email firstname.lastname@example.org. Her husband is also listed as providing driving services so it appears if you combine the two they may provide just what you are looking for. If you Google "Alma Elezovic tour guide" you will come up with a Facebook link and links to past RS posts where you can get more information.
I'm sorry, in reading your post again I now see you may be looking for someone who will take you from Dubrovnik to Mostar. I think the service Alma provides is once you arrive in or near Mostar.