A number of custom made straps have been posted to this thread in the past. As indicated, best to find a cobbler or someone used to working with leather - saddle/tack maker, etc.
Just found an example of custom strap on the first page of this thread
Moving away from a leather strap. Filson make a nylon strap for some of their bags, it may be possible to buy one of these.
There are also some less dressy solutions, such as op/tech's SOS (Save on Shoulders) straps.