The two best sources I've seen for finding parts for player pianos are

1. eBay auction: If you wait long enough, anything you want will come
up.  Right now there's a Welte pump, motor and stack, and a complete
set of parts from an 88-note player.  Regularly you can find the entire
top action, etc.

2. MMDigest: You guys are all great.

If you are interested in a Tom Thumb you're going to have trouble
finding exactly what you need on eBay, but maybe you can take parts
from a standard 88-note and modify them to what you will need.  You
would need a tracker bar from a Recordo, though.  You might have to
wait a long time until one shows up on eBay.

Please don't attack me for suggesting replacing parts in a Tom Thumb
with parts from an 88 -- I don't know if they are compatible, it's only
a suggestion.

Eliyahu Shahar