I went with imperials at first so i could get the jagged crown from Hadvar after the jagged crown quest, However after getting the crown i decided to give it to ulfric. Now i cant find where Hadvar is so i can pickpocket the real jagged crown. Why is this?

Are you on the PC? Do you have the unofficial patch?

IIRC once you join a particular side for good - i.e., bring the Jagged Crown to either Tullius or Ulfric - your "Helgen buddy" from the other side of the conflict disappears from the game. If you brought the Crown to Ulfric, you may see Ralof again on some of the CW missions that follow and also back in Riverwood after his part in the war is over (although some discussion at UESP indicates otherwise) but you won't see Hadvar. He is out fighting for the Legion and you will not cross paths again during the war. Afterwards he is apparently presumed dead in the conflict, or has withdrawn with whatever is left of the Legion. As far as I know the continuing presence of Hadvar or Ralof in the game is controlled by which side you end up joining; having followed one or the other out of Helgen will not keep either one of them in the game once you've joined the other side permanently.

ah, so its impossible for me to get the crown and join the stormcloaks... because i went with hadvar in the beginning. Lame thanks for the info

no i dont :/