I think ESPN has something on the ballparks, but that only runs throughout the season I believe.
You just have to realize that ballparks, although influential, will not make or break a guy. Only Coors Field has the ability to skew stats with any significance, and even that doesn't help a guy who's not a power threat to begin with. When you consider pitching a guy in Comerica or Safeco, or hitting a guy in Minute Maid, remember that it's just as likely that there will be no improvement at all. look at the overall picture.
What does help though, is more specific splits. For example, New York is not considered a top hitters park, but that short porch in right field is tantalizing for lefties. Knowing that, I picked up Travis Lee for one game last year, because I knew that he had insane numbers off of the righty, with that grandstand just over 300 feet away. Low and behold, Lee went 3/4 with a HR and helped me get a lead that I wouldn't relinquish.
It's that type of stuff that's usually more important. Don't start a bench player against Mark Prior at Minue Maid just because he's got a great home split at Minute Maid. Consider ballparks just one more piece of the gigantic puzzle.