My horse is sensitive in his back and I use a Bartl sheepskin numnah, which has helped no end. It's nice because it has air holes along the back. It was a really reasonable price as well.
Sheepskin ones are very good but even the best go bit manky eventually so before you put them in the wash we smother the sheepskin in conditioner and rub it right down to the roots, and brush them out with a soft brush when there dry. x