Further Odd Keystones can be obtained from the captain of the S.S. Note that the keystone is consumed immediately, so defeating the Spiritomb will not let you immediately retry without another Odd Keystone. You will be asked if you want to throw the keystone into the well choose Yes to encounter a Level 20 Spiritomb. With a keystone in hand, return to Azalea Town and stand on the top of the ladder into Slowpoke Well, and press A. The only thing you need to get this Pokémon is an Odd Keystone Cynthia will give you one as you are on your way to Goldenrod City.
Note that if you wish to change Rotom’s form prior to reaching Saffron City properly, there is a thief in the bottom of the Goldenrod Mart behind a box who will teleport you to the appropriate room. You will be asked if you want to thump it, á la Diamond & Pearl choose Yes to encounter a Level 20 Rotom. Obtaining Rotom is fairly simple once you earn your Plain Badge from Whitney, return to New Bark Town and interact with the TV on the bottom floor of your house.
