OP, are you looking for an English saddle or a Western saddle?
If an English saddle, I'd go for a wide or medium-wide Stubben. They tend to better for "dippy" backed horses. You can find used Stubbens at a good price. I'm sure there are other brands out there, Stubbens are just the brand that I have the most experience with that would likely work for you.
For a western saddle, you want a short, round skirt saddle with lots of clearance under the pommel. A barrel saddle might be a good choice as they tend to be shorter in length and have good clearance under the front. You won't know if it fits until you try it though.
It would be best to work with a local tack shop that can give you saddles to try out. If you don't have one locally, then you should find an online tack shop that can help you do wither tracings and measuremtns, and look at photos to narrow down your saddle choices to saddles that will likely fit.