Where do you get Dialga in Pokemon platinum?

Answer 1

You get it at spear pillar after defeating the elite 4, talking to the old lady in celestic town and getting the adamant orb which is up the waterfall in mt. coronet.

talk to chnthias grandmother after you pass the game and go back up to the spear pillar and there is two portals.1of them is pink which is palkias and the other 1 is blue which is dialgas both Pokemon are lvl 70 so be prepared