Are you looking for western or english?
I agree, an Arabian tree would be the first place I looked. They tend to have more flare to the front, which allowed for freedom of shoulder movement.
Kevin and Southern Trails is a great guy to work with. He has his own line of saddles as well as other good names brands. He WILL find one that will work for you
. You can do wither tracings and send pictures, to give him a better idea of what will work. All of his saddles are reasonably priced too, and nice quality. Western Saddles, Trail Saddles, Barrel Saddle, Ranch Saddle Western Saddles, Trail Saddles, Wade Saddles, Ranch Saddles
If you're looking for English saddles, I would try a 30-32 cm Stubben Siegfried or medium/regular tree Passier. Both have fit the Arabs and half-Arabians I have worked with quite well.
With any saddle, buy one a little wide and use extra padding to fill in, so her muscle has room to grow. You can adjust or swap out pads as her topline improves. I like Skito saddle pads
best for improving saddle fit and muscle development. They're very versatile and comfortable for the horse. You can order custom shims for only $15-25 a set to fix almost any saddle issue. They wear well too, lasting a very long time. My oldest is over 8 yrs old, and going strong!