Brew Vendors sell the beers players get from the Brew of the Month Club. You get one sample beer mailed to you each month, and you can buy additional beers from this vendor, but only for the current month. You must be a member of the Brew of the Month Club to buy from this vendor. The beers come in stacks of six and stack up to 24 in a stack.
See and Brewfest for more info about the Brew of the Month Club.
These two NPC's can be found:
Alliance: Standing next to <Brew of the Month Club> in the Stonefire Tavern near the entrance to Ironforge, on the outer wall between the Auction House and The Mystic Ward. Stonefire Tavern is marked by a mailbox out front.
They are each friendly to the faction in which they are located. They may be friendly to the opposing faction as well and you might be able to use the Brew Vendor for the opposing faction if you are willing to fight your way in.