Gen III --> Gen IV --> Gen V
The only way to obtain it is via poketransfer ( Getting one from your old games ) or the Dream World. And it's not available via DW yet.
So you would have to get one on a previous game.
HG / SS: Bug catching contest ( After obtaining national dex )
D / P: Eterna Forest
Platinum: Eterna Forest ( Pokeradar )
R / S / E: Route 116
How to use poketransfer: http://pokemondb.net/pokebase/54877/how-do-you-use-poke-transfer ( That's from HG / SS or D/P/Plat to B/W )
How to migrate: http://pokemondb.net/pokebase/877/would-migrate-pokemon-emerald-version-into-game-like-diamond ( R/S/E to D/P/Plat or HG / SS )