I saw that forum post about the 3x 18-slot bags too, but I'm not aware of any such option.
buying them from the TP, but the cost doesn't vary much regardless.
There are a few variations along the way, such as obtaining materials yourself vs. Otherwise there's only one method to get any >20-slot bag, which is to craft it yourself from a 20-slot bag.
As far as I know, the only >20-slot bag you can get from an achievement is the Reinforced Olmakhan Bandolier (32-slot bag from LW 4.2, Sandswept Isles).
