I have a VERY VERY high withered Thoroughbred. I talked to a Custom Saddle maker who does saddle fitting and he understands that everyone can't buy a saddle for every horse they own.
I have a Circle Y Show saddle that I bought for $500, very nice saddle and LOVE it! It has Semi-Quarter Bars in it which most Thoroughbreds need. We were still having issues because it sat about 1/2 inch above her withers so her withers kept hitting it. I had a 1" thick neoprene saddle pad, so I cut it to make a cutback like her suggested and we had improvement instantly! So I went and bought a really nice wool saddle pad that was cutback.
I recently bought a new barrel saddle, it fits but it is a little to wide so I have to double pad it and then it will fit better.
This is where I got my saddle pad, it is 1" thick, very good brand. Thrifty Horse - Wool Pad with Wear Leathers