Where can I find a challenger II barrel? or does an aftermarket one not exist yet?

Herchealer
To the best of my knowledge there are no aftermarket barrels available for the Chally 2,the only place you could look is the Accurate Armour update set, I don't know if there is a barrel in there.Or you could just contact AA and ask to buy a barrel from their chally 2 kit.
HTH
Pete

Yes, the AA update set or conversion set includes a barrel. The "original" Trumpeter kit one is too small and too short, not to mentioned that the thermal guard details are not realistic at all. (Trumpeter has corrected many errors with the newer issues of the Chally2, but I am not sure have they corrected the barrel)

You can always ask AA for some parts in the kits. They will quote to you separately. They are one of the best online shop out there.

But many small details of the Trumpeter kits are not that good. A full AA update set will make your model look much better.

Thanks for the Awesome Ideas Storm and Yorkie