You can get a new one from
HorizonWirelessOnline.com - Horizon Wireless, The World’s Cellular Repair Center ß. They are great to do business with (and also a site sponsor).
To replace the ball and bezel, use a very (very) small, flat screwdriver to gently pry up the bezel. Both the bezel and ball will come out. If you have compressed air, blow a short burst into the hold to get any dirt/dust/yuck out of the hole.
To put the new ball in place, first look a the bottom of the trackball - there is a shape with a 'leg' pointing one direction. Make sure when you drop the trackball into the hole, the 'leg' is pointing towards the space bar.
For the bezel, there are 3 prongs on it. Make sure the prongs are at the bottom, left, and right of the hole. Then pop it into place.
I've had better luck if I gently push down on the right side of the bezel just a little harder then the left when pushing it down into place.
Good luck.