They're mostly made to order, they have trees with fairly wide and flat rails but none are super flat and it also depends how well the panel is constructed as to whether it will fit - cobs vary massively in shape (I use three specialist but quite different variations on wide and flat to fit these shapes, plus one more specialist dressage one). Can you work with a good fitter?
Ive got an older Farrington Dressage, it looks like the Derby House Kentucky, it was used on a very high withered horse, the reason it was bought as nothing else fitted. I kept it for my young horse, as worth more to me than its value and I loved the deep seat and knee rolls sadly the saddler said the tree is too flat for her. I always thought because it was such a deep seat, the tree would be curvy lol but it is actually very flat along the back when I look.