I know!go to the valor lakefront then use your dowsing machine{poketch app}then you will find 2 items somewhere first is the suite key second is the sun stone so good luck
Were Can You Find A Sun Stone In Platinum?
Lol, I have never gotten one in the Underground, but I hear thats where they're at.
You don't! You can only find them in Diamond and Pearl! Sorry for your troubles about the Sun Stone!
All you have to do is dig one up from the underground. Or just get one from your diamond or pearl.