My OTSB has VERY high withers and the only place I could find that carries saddles that are made just for horses like that was dover with their thorowgood saddles. Is there any other saddles (preferably dressage, but AP is ok too) out there that are built like that?
Look for a saddle with a cutback pommel. Some brands with models that will fit the bill include Passier, Kieffer, some Stubbens, some Crosbys, some Collegiates, etc.
If that's you and your horse in your avatar, you need to move your saddle 2-3" farther back. The front points of the tree (just under the front d-rings) should be 2-3" behind the back edge of the horse's shoulder blade.
Fit the saddle to your horse's shoulder. The high wither fit comes from a cutback pommel.
Wow. That's pretty high... Can you borrow any saddles around or in tack shops to try? She's not typical build, so personally I'd be really careful to buy anything on Internet (at least if you can't return it back). I still think diginary probably will give clearance on her withers. I know places like Dover and I believe Valley Vet and Jeffers Equiene can give you "trial" on saddle.
Friend of mine have thorowood(spell? :) ) for her wide draft and frankly it looks very low on withers. In fact I was thinking about looking into one for my other horse (low withers, wide back). She has A/P though, so dressage may be different...