If they're high-withered because of a lack of topline (OTTB, young horse, etc.), I would recommend a Thorowgood saddle until the topline has developed, that way you don't break the bank on something you'll only ride in for a year or so, depending on their progression. I'm personally not a fan of adjustable trees. If the topline exists and they just have shark fin withers, I'm always a supporter of County.
Passier. They have adjustable trees and cut back pommels. Best purchase of a saddle having bought quite a few for TB with high withers.
I really like County Saddles for Hunter/Jumper and Hunter Eq. - I have a high withered Apendix QH and it fits him perfectly (very little customization to flocking)