Hmmmm...I would have to take apart my trackballs, but from what I
remember, these are probably slotted photointerruptors. You might do a
search on Mouser.com and see what you can find but you probably need to
know the specs to get the right one. By the time you factor in the time
spent trying to find just the optics and then the time to solder them
in, plus the cost, $16 for the whole thing looks pretty good to me. ;-)