No, there isn't. The legendary Pokemon for emerald is raquaza and that's it. when you finish the Pokemon league, you finish the game.
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Emerald Gardens for sure ;)
Only by a Nintendo event.
There is no town map. You might be referring to the Pokenav, which is Ruby, Sapphire and Emerald's version of the Town Map. You can get it at Rustboro City from the Devon Corporation's President.
The Old Sea Map was an event item that allowed you to access Faraway Island where Mew resides.