To get a Dratini on Emerald, you must have
a copy of either PKMN FireRed or LeafGreen and
a gamelink cable.
Purchase a Dratini from the slots in FR/LG and
trade it over to Emerald.
Dritini can be found in the Safari Zone. In area four (Near the place you get your prize; HM Surf) use a Great/Good Rod or go Surfing. You'll eventually run into a wild Dratini.