You can find those around the Grikbar Crater ( -284, 84, 1159 ) /waypoint -284, 84, 1159 or the Mazkeen Crater ( -743, 130, 669 ) /waypoint -743, 130, 669. These are big black rocks and very easy to find. They respawn quickly so it is perfectly possible to get all 15 at one crater.
Kill the crazed kobold at Mazkeen Crater in Steamfont Mountains. This is a solo named found near Mazkeen Crater. His name is Grak Shattersnout (35^) ( -813, 154, 661 ) /waypoint -813, 154, 661, and he will spawn on the Northern edge of the crater as you approach (will not immediately aggro if you are invisible).