Spiritomb is an incredibly unique Pokémon. Being of the Dark/Ghost type like Sableye, it has no weaknesses. This page deals with the method used to obtain Spiritomb, one of the most long-winded ways to get a Pokémon:

The first thing that you need to do is get a Odd Keystone. You may find one lying about in Sinnoh, but it'ld be easier to find it in the Sinnoh Underground while digging, although it will be a bit rare.

Take the Odd Keystone to the Spirit Tower Well in Route 209 and Place it inside the Well. This will activate the event for Spiritomb

Next you need to go to the Sinnoh Underground and talk to 32 People there in total. These do not include the Hikers that give you items.

While this does sound like a Daunting Task, what you can do is exit and enter 32 times and talk to the same person. Each time this will count as talking to a different person so although it will take time, it can be done as long as you have a friend with Diamond & Pearl. Remember the Sinnoh Underground is DS Wireless, not WiFi so you need to physically be near your friend in order to do this

Now with 32 people talked to, go back to the Spirit Tower Well in Route 209 and you will battle a Wild Spiritomb. It is fairly low level so be weary.

You can do this multiple times as you can get multiple Odd Keystones but it can get a bit long winded so it may be best just to breed to get that perfect Spiritomb