Mining Sack

Source
This item can be purchased for from almost any Bag Vendor (and some random other NPCs) in any major city as well as the following less-obvious places:
 * in Durotar
 * in Hellfire Peninsula
 * in Hellfire Peninsula
 * in Shattrath City
 * in Netherstorm

For a full list of who sells this bag and where, please check the Wowhead link below.