Odd Keystone

You can find them in the underground... When you dig in the walls. They are somewhat rare, so keep it up. Simple as that. If you want spiritomb, then you will need to visit 32 people in the underground. (32 other people, not the hikers/sellers under there) they can be the same person 32 times.

I think they may be one of the items a Pokemon with 'pickup' ability will pick up around sinnoh... But I'm not sure.

You can also find one in Hearthome city. Don't aske me where, it's been ages since I have done it.

They can be found by digging in the Sinnoh Underground, and one can be found with the Dowsing Machine in Twinleaf Town. A Black Belt just west of Hearthome City has one too.
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Eonteam123456

You can obtain an Odd Keystone from a black belt on the route west of Hearthome City. Go in the bottom left corner near the grass patch, and talk to him. He'll give it to you.

Also, if you use the Dowsing Machine in Twinleaf Town near the water, you should find one.

And finally, you can VERY RARELY find Odd Keystones underground.

either go 2 twinleaf town and surf look in between the left side of the trees and theres 1. or go to hearthome city and go left, if you look up u will c the berry masterz house. go down and u will c a karate man. talk to him and he will give you the stone. go then south of solaceon town or east of heart home and click on the wierd rock thing. put in the keystone and thats done. now you have to get a friend and go underground and talk to him 32 (i do it 33 times) times differently. each time you talk, go up and then go back under and talk again. after youve talked 32 times, click on the Hallowed tower and fight spiritomb. i suggest using timer/ultra balls.

It can be found underground, a black belt ninja, or by going to twinleaf town, go in the middle of the lake, and go to the far left and push a.