On ebay now
Military Tank Track Track Jack Track Fixture 12252120
Pic of the more modern Track jack on ebay for buy it now of $59.50.
When I bought one for my first carrier it was $25.oo
Anyway, if you get one of these, You will need to grind the horns to fit in the carrier track. If you don't, you will have the track together and pinned and then not be able to drop the jack away from it.
Speaking from experience!