Question

Where do I find a Thunderstone in Pokemon Platinum?

Answer 1

Non-repeatable

• Solacean Ruins (On the rock in the dead-end down the top right, bottom left, top right, top left, and then bottom right stairs, hidden)
• Sunyshore City (Behind the Vista Lighthouse)
• Route 229 (By the southwest end of the pond, requiring Cut and Surf, hidden)

Repeatable

• Underground

Thunder Stone (Bulbapedia)

Answer 2

Route 229, Solaceon Ruins (With Dowsing Machine), Sunyshore City
Hope I helped!
Source: http://www.serebii.net/itemdex/thunderstone.shtml and experience

Answer 3

There's a thunderstone on the southeast side of Sunnyshore City near a rock (the munchlax one I think)
Also theres one in Solaceon Ruins, or you can go into the underground and spend the next couple hours looking for one. =)