does this exist in aftermarket?

Don't know if such AM are available but here is one chieftain kit with individual track links

http://www.internetmodeler.com/2002/march/armor/chieftain.htm

Try these guys: http://www.accurate-armour.com/

They make resin track links. I am not sure if they make the ones that you are looking for though. They also have lots of conversions for the Chieftain that you may want to check out. I think you can order directly from them, or you can order through Great Models. Good luck in your search.

Fruil Model makes tracks for the Churchhill and Centurion but not for the Chieftain. ModelKasten makes tracks for those 2 as well, but not for the Chieftain. I am not one for British Armor, but even though the Chieftain tracks are similar in design as the Churchhill, I believe the Chieftain had rubber pads on the tracks.