Black 2 White 2
Trade in Nimbasa City
It's the dropped item event. You speak to either Yancy/ Curtis, and after enough calls they will offer to trade with you.
Also, you can't find Spiritomb in Dream World if you use a B2W2 account; you need use a BW account to find one in DW. So if you have a previous game, then you could transfer one. Otherwise, Yancy/ Curtis is the only way to get a Spiritomb in game.
Trading with another player is the other option.
Dropped Item event