The Odd Incense can be found in the Cerulean Cave along with Mewtwo himself.
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Breed Mr.mime while holding an odd incense
at the cerulean cave
You can't.
You can't. The Odd Keystone is only obtainable in Diamond, Pearl and Platinum. You will need to trade a Pokemon with an Odd Keystone to get it on SoulSilver.
in cerulean cave