It depends upon how much land you want and how much you intend to ride. There are pockets on homes with horses all over Boise and the Burbs ... with the exception of big subdivisions and the heart of downtown ... its seems like someone has horse property every couple of miles in any direction.
If you wany more land and more neighbors with horses .. stick to the outskirts of town - any town - boise, Meridian, Star, Eagle and Kuna. If you want areas in which to ride, I would go North between Eagle and Star or South between Meridian and Kuna.
Check out Google earth and you can clearly see where the propterties are with lots of space and horses.