Check out Thorowgood saddles. Dover Saddlery carries them and you can sometimes find them on ebay
. I believe they come in 18". I have a 17 1/2" and it's a little roomier than my Courbette 17 1/2". They sell new for about $550 I think.
Thorowgood makes a saddle specifically for high withered horses. The T6 looks like a leather saddle, and I believe you can get it in close contact. They also have the adjustable gullet change system as well as inserts to fine tune the fit just a hair.
The under panels on the high wither saddle are a little wider, a little further apart than a standard saddle, to give the spine more clearance.
I got a high wither old Thorowgood for my mare, and she has been much more relaxed under saddle, no more tantrums, or crabby faces. My saddler adjusted my old saddle numerous times, said it fit her great, but I could tell my mare didn't think the same as he did. http://www.thorowgood.com/
Click on United Kingdom, then click on Product Range. Then you can read about the T6, T4, Maxum, and the accessories.