Another vote for Backwoods Leather. I travelled from Richmond because he both could and would make a custom holster - and then he suggested a slight change that made it even better.
Depending on the type of alteration you are wanting, your local shoe repair shop might be able to do it for you. I had a holster that the belt loop was "too big". My shoe repair guy put a few rows of stitching in and not only was it then "just right", the Three Bears never even showed up.
He's also repaired stitching that "wore out", and put in a row of stitches to increase the retention on a third holster. (The best part is he & his wife are Koreans who keep supplying me with kim chee when I come to pick up my holsters. Well, that and I get to see the faces of the Carytown ladies who are coming in to pick up their shoes with the new soles & heels or the whole high heel put back on when they see a holster passed over the counter.)
stay safe.