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.