If you're unfamiliar with this sort of drop, a good example is the Hearts of Darkness from Mount Hyjal and Black Temple. They drop off of trash mobs in the raid instances, and are fairly common. The Hearts of Darkness in BT/Hyjal were used in resistance gear for Mother Shahraz as well, so we may still run into some of that in the Sunwell. However, if we don't, expect Sunmotes to be a little on the rarer side of the spectrum since we won't need quite as many.
Sunmotes don't seem to be Bind on Pickup either, so if you don't have a 25-man raid group, there's still a chance you can buy these from an established raid group once they have extras.
My thanks goes out to World of Raids for the information and screencap!