This should be posted on the "Horn Bows" page on this sight. I've got a few bighorn bows made in the last few years posted there as well as a couple other members.
Rocky mt, desert or California bighorn works best. Alaskan dall or stone sheep work, but have more twist to deal with. Mouflon sheep works, but is thinner and harder to get a heavy bow. Trophy horns will almost always have good horn to work with under the scuffs and make a longer bow. They also cost more.The biggest concern is beetle damage. Look for little holes in the horn. Also look for a smooth, undamaged inner surface. $4-600 is about right.
Good luck