You can get the stone if you go to Route 18. After meeting Crasher Wake(if you haven't already), go through the grass patch and across the bridge(you could battle the veteran, but you don't have to) and you will see a pokeball. It should be the Magma Stone. If you got it already, fly to Reversal Mountain and go to the deepest part. Heatran will fall from the sky. It is level 68, and is surprisingly easy to catch. Hope this helps.
