Okay so since Blizz has not updated easily crafted bags since netherweave. Since Tailors have to farm to make anything larger and they cost an inane about of mats. Since I think spending 200-500g per 22 slot embersilk is retarded. I have compiled a small list for people who like to roll alot of alts.
Tattered Hexcloth Bag
24 slot bag -Zul Aman, You get this for completing the dungeon quests. (85 Cata Heroic)
Grummlepack
24 slot - Kun Lei Summit, part of the Grummle quest chain it is a reward.
Dragon Hide Bag
22 slot - OS, Drops of Sarth. (80 Wotlk raid)
Enlarged Onyxia Hide Backpack
22 slot - Onyxia's Lair, Drops off Onyxia (80 Wotlk raid)
There ya go. Free upgrades. Largest you can get for free.
There is also another 22 slot alternative in the LK heroics but it is a chance drop and all these are 100%