I don’t know where you are located but I would look up local saddle fitters. They will come out and fit your horse to the T and then suggest saddles for you to look at to purchase or they might have one for you to buy.
My horse had an issue where he needed a MW tree but with panels since he really is a Medium tree. Most of my saddles would sink down because they ended up being too wide so my mistake was to think he needed a narrower tree which in the end ended up hurting his back. I had a saddle fitter come out and he explained I needed a MW tree but with panels underneath to help fit snugly around him. It fits perfect! He’s slightly downhill as well.
My saddle for my odd fit was: Black Country Saddle:
Also, if you can find a used Harry Dabbs jump saddle or all purpose they tend to fit a ton of backs! I have one and it fits almost every horse I put it on.