This item is also currently banned from competitive play.
 Locations Found
Soul Dew is attached to the Wild Latias or Latios that appears on Southern Island in Ruby, Sapphire and Emerald. However, this area is only accessible by way of a Nintendo Event that gives out the Eon Ticket, the item that accesses Southern Island.
As the result of a Nintendo Event, you'll receive an Enigma Stone. Take it to the Pewter Museum in Pewter City, it'll be discovered to be the Soul Dew and returned to you. The specific Pokémon that appears as a result depends on the game.
In Heart Gold, this will cause Latios to appear; Latias will appear in Soul Silver.
A Soul Dew can be purchased for 14,400 points, the only game in which a Soul Dew can be purchased as of writing.
Soul Dew is unavailable in Generation V as of writing.