How do you get a thunderstone in Pokemon Diamond?

You can dig underground for thunderstones, but you can also get one in Sunyshore City. That one is located near the house with the guy who wants to see a certain nature of a pokemon. Like timid, and brave, and whatnot.

Before you get the national dex the thunderstone will be very rare in Diomond and Pearl but after you get the national DEX it will become normally rare in Pearl and uncommen in Diomond so youll have a better chance after the national DEx in Diomand and Pearl But youll find more thunderstones in Diomond..............youll find it in Sunyshore city, but more importantlly .......UNDERGROUND...........!

For more info about any Pokemon game try www.psypokes.com/dp/items.php

Hope this helps!

1) Migrate a Pokemon from a GBA game holding a thunder stone
2) You can find them by digging in the underground
3) You can find one in Sunnyshore City