Sold through Blizzard's official store. Physical copies seem to be sold out, but you can buy the digital versions.
$5 for base game
$5 for Wrath of the Lich King
$10 for Catalcysm
Offer ends August 27th.
http://us.blizzard.com/store/browse.xml?f=p:110000044,p:1100000265,p:1100001289