If you do a google search on MK4021GAS (40gig) you will get a lot of returns. You can get new or used ones and they range from 40 to 80 dollars. You can also go to the MK8021GAS (80gig) for about the same price range. When you take the hard drive out you will see a black adapter on the hard drive pins. You will have to remove that and use it on the new drive. Other then that it is a straight forward swap out.
Have a nice day,
Denis