The only way to obtain them legitimately is to complete the Hoenn Pokedex (after which Prof. Birch will have them in his lab) or get their evolved forms from Pokemon Colosseum (gamecube) and trade them to Emerald/Ruby/Sapphire which can be quite complicated. The easiest way to get them at this point in time is on Heartgold and Soulsilver.
Believe it or not, it's possible to get one of the Johto Starters in Emerald. Yup, you heard me right. There's a way of getting either Chikorita, Cyndaquil or Totodile in Emerald, WITHOUT being a dirty, rotten cheater or trading from Colosseum.-PKMN.net
Only for Emerald sorry
For more info go here- http://pkmn.net/?action=content&page=viewpage&id=5514
For Sapphire and Ruby, sorry there is only one way - Trade from Emerald/XD
Hope this helps =P
