lots of good ones here:
deviantart.com
just search for Paul Pierce or the Celtics. pages apon pages of them.
Not to be a jerk or anything but you really shouldn't just be hopping on deviant art and grabbing peoples artwork and using it as your own.
As someone who has done a good deal of graphic art, I would suggest getting/making your art in some other manner.
However if you can get permission from the creator, its all good