Hey Stang.
If you don't mind teh strap ( good for retention) you can try to strectch the leather starp with saddle soap, leather conditioner and such.
Depends on the leather itself as well as it's finish.
Leather workers are getting to be a rare breed. Best bets are...
1) Old time shoe sotre guys (they usually also make/sell keys

).
2) Check with horse people. They are always getting saddles, reigns, bits and spurs repaired.

3) Or you can alway's check with your local BDSM types.

For the do it your self, is the strap integrated into the holster body or is the strap sewn to it ?
Photo ?