Otherwise go with either Armorsmith or Tailor, because Leatherworking materials are more expensive.
Some of them have different sub-types for different crafting disciplines, in which case choose whichever one you've already gotten to level 500 on some toon, or plan to. Craft or buy the 20-slot version you want (that is, whichever one has the properties you want-generic, materials/upgrades, equipment, junk, invisibility, or containers).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).