This is a picture of the saddle. I have a traditional rectangular saddle pad for it that fits OK - all of the leather on the saddle is on the pad without too much margin of the pad underneath. I know there are better pads on the market, though, and feel like I might need to get something more suited for this saddle and comfortable for my horse. I'm completely overwhelmed with what is available, though.
Go to horsesaddleshop.com. Look at the tucker cool back and Southwest coolback pads. That store gives exact measurements of the pads and they are pretty good pads without being insanely expensive. Run about $80. I ride in a large australian and took me awhile to get a pad that fits.