You would probably have to buy a whole new trackball assembly. I believe this is the
OEM trackball.
This is the new
Atomic Trackball, its black and doesn't light up, however it is the upgrade to the older trackballs. Supposed to keep dust out of the Track Ball cavity. I cannot vouch or either since I have been forunate enough to not be confronted with this problem.
Good Luck,
Hank
P.S I only suggest these links because when I order from them the items came with installation intructions.