Celebi was given out at GameStop Locations in North America (http://www.serebii.net/events/2011.shtml).Darn, how do i get a celebi?
btw, i do own a HG
The other way is to obtain a Zoaroak and breed it. In order to obtain Zoaroak you need to transfer one of the Shiny Beasts from HG/SS to get a Zoaroak which you can then breed to get a Zorua. These were also event Pokemon via Gamestop locations or the Nintendo WFC