I purchased my new trackball from blackberrytrackballs.com for $13 USD.
Came in about 3-4 days. New ball went in well and provided a nice feedback that it was moving around. The original ball was smooth and was redish looking in comparison to a new one.
PIN: 23E23179 - T-MO - Pearl 8100 - Retired
Android G1 - T-MO