The Ballroom Expansion Set at DAZ includes an elegant grand piano and matching piano bench. But I doubt if the keys are movable - it is a prop not a character. If you are a Platinum member at DAZ the set is $1.99 but be sure to get the Ballroom, too for another $1.99.

If I were making a piano pose I would find some pictures of concert pianists on the Internet, start out with a basic seated pose and then pose the collars, shoulders and forarms to get a position that looked like the photo and do the hands and fingers last.