I found that, primarily a pair of golden ACG was collectable from Rewards Track during Showdown in the Badlands.
However, it has been changed to one of the rewards from an achievement which require to play 200 Quickdraw cards. (probably since Whizbang)
The problem is, there is only a single copy of golden ACG available in the rewards. Another golden copy is never collectable, neither from crafting nor any sort of rewards.
I believe that this is unintended and considered as a bug.