Today only as per Blizzards news article you can purchase and lauch fireworks from vendors in the major cities.

KABOOM!
Deep within the cavernous halls of Undermine, goblins labor and toil over their strange and fanciful engineering creations. When they are not constructing powerful devices of destruction, they find making fireworks a diverting pasttime. They work tirelessly over strange chemical concoctions used to produce precisely the right color and, of course, spectacular explosion. It is an artform that comes naturally to those who find great pleasure in making things explode in a variety of ways. Competition is fierce among the engineers as they strive for perfection within their pyrotechnics.
Since holding an exhibition indoors -- particularly within range of a goblin engineer's workshop -- is completely out of the question, it was decided by the Tinker's Union that a day would be chosen wherein the pyrotechnists would go out into the world and share their creations with the world (for a modest fee, of course). For one glorious day, the sky would be alight with the fantastic creations of engineers throughout Azeroth.

Whether Alliance or Horde, these pyrotechnists are more than happy to help you set the sky on fire during the Engineers' Explosive Extravaganza! (Just remember, point all incendiaries away from the face.)