How can you get a spiritomb on Pokemon platinum?

you have to go to the hallowed tower. the Hallowed Toweris a crumbling tower located on Route 209. On it is an inscription that is unreadable until the Odd Keystone is placed on it. It is the only location of Spiritomb.

To get Spiritomb, the player must:

1. Go to Route 209 and place an Odd Keystone in the tower.
2. Go into the Underground and talk to other players a total of 32 times.
3. Return to the tower. A level 25 Spiritomb will appear and a battle with it will begin immediately.

It is important to note that one does not need to talk to 32 different players (it can be the same person 32 times, 32 different people, or a combination of both). However, one of the players may have to leave the main part of the Underground between interactions for each one to count. The fastest way to accomplish this is by entering and leaving a secret base; alternately, digging in the walls for treasure between each interaction also works.

Players refers to actual people playing the game, who appear in the Underground via DS Wireless Connection. Talking to the NPC vendors will not count towards the total needed to summon Spiritomb.

Spiritomb can be summoned again; however, all of the steps listed above must be repeated to do so (including talking to players underground another 32 times).

If the tower is interacted with after placing an Odd Keystone, but prior to talking to 32 players in the Underground, the game will display various messages, such as, "...Is that crying coming from inside the tower?

or you can trade for one at the GTS