a good saddler should be able to adjust the flocking, or you could send it back to Ideal with a template if it needs different shaped panels, for example. my high-withered mare's in 2 Ideal saddles which have a front gusset in the panels to make the saddle sit on the top of her ribs, your horse might need something similar.
I have a black country saddle for my high-withered trakehner. It took a lot of searching to find one that would fit but eventually my saddler found this black country one that is designed for high-withered horses. She did have to re-flock it but now it fits well. I bought it second-hand for £475.
Have a look at their website - I find mine very comfy.
Good luck!