There is a way to get it on Diamond/Pearl. First of all one of your friends must have Platinum, then help them get the Griseous orb. What you need to do is make your Platinum friend make one of there Pokemon hold the Griseous orb that thy don't want. Then trade with the Pokemon thats holding the Griseous orb. TA-DAH!
Griseous orb is held by Giratina, and makes it into its original form. Unfortunately you need platinum to get it. Its in turn-back cave after you beat the elite four. It is unobtainable in Diamond and Pearl, and from what I have heard, you can't even trade it to diamond and pearl from platinum
If you mean the one Dialga is supposed to be holding, it will be where he was standing on the Spear Pillar, I got frustrated when I heard that, but it was worth it though! Haha but if that's what you mean though, but if not, I'm sorry, I can't help you! Good luck finding it though!