Here's a great way to help determine if the front of the saddle will fit your horse: http://www.yourredwagon.com/Degree%20Method.pdf
Once you get the saddle, here are some good saddle fitting guides: Saddles That Fit! Step 1: Saddle Position Treeless Saddle Fitting Guide and Saddle Fit Help
The tree has to fit more than just the front of the horse.
I would personally recommend an older Stubben 30-31 cm tree or Passier Medium tree saddle. Both will fit a wide variety of horses because of the short tree points. Or, if you don't mind synthetic, then a Wintec 500 or 2000 would be good choices, as they have a changeable gullet that will fit a wide variety of horses.
I would also recommend getting saddles from an online consignemnt shop that allows trials, like usedsaddles.com or saddlesource.com.