Anybody who does barrel work should be able to contour, thread, and chamber the barrel from a blank.
Greg Tannel would be another name. I bought my through-spindle chambering lubrication system from him. Seems to be a good guy.
E. R. Shaw makes a less expensive line of Savage replacement barrels that Brownells sells. You could contact them about doing a special. They already do .375 RUM with a 12" twist.
If you want to save some money by selecting the grade, call
Douglas. You can order any twist and grade, and for a fee they will rough contour it, and for a bigger fee they will polish and finish it, and for a bigger fee, chamber and install it.
The left hand twist is the only fly in the ointment. I don't know which barrel makers have gear that will work in both directions and which do not. Is there a particular reason you want that? The only one I can think of is to have bullet precession and northern hemisphere Coriolis effect working in opposite directions at extremely long ranges.