Were to get the last badge Pokemon platinum?

Answer 1

sunyshore city, but first you need to go to the peak of mt coronet and catch palkia because the man near sunyshore will tell you there was a blackout and wont let you past unless you have palkia.

P-S, you cant just trade a palkia from another game, you need to do the event where cyrus and the rest of team galactic are there trying to use palkia. so catch palkia yourself!!