Need help fitting a saddle for a high withered thin built Tennessee Walker!?

I have a 10yr old, high withered and thin built Tennessee Walker gelding. I need some ideas from someone who has a horse like this on what kind of saddle to use. I cannot afford a custom made saddle. My current saddle works, but slides back from his high withers and all his natural shoulder movements. I have researched a flex tree or treeless saddle. I have heard positives and negatives about both depending on the brand. I have also researched corrective saddle pads. I don't want to go that route. I feel they just cover the problem of an improper fitting saddle. Can anyone please help me that has experience with this shape of horse?