If forced, the only place south of Mammoth I would go is the Sport Chalet in La Canada (provided the original ski shop is still open).
As of a few years ago there the original shop was still there and it was only Ski stuff and all the dudes that worked there were people that had been working there for ages. If it's not still there....
There are some shops that cary decent selections down south... but I wouldn't trust any of the boot fitters with my feet. There used to be a guy at Val Surf in Valley Village, but I think he left.
Then I'd make your way to Mammoth or Tahoe.