Yes if you haven't already carry out the tasks which the Underground Man assigns you, and check down in the Underground in walls. They are common enough to find really, I have about 8 of them at the moment and I only started going underground a few days ago.
Just as they said underground is the place to look. Also, if you dont like chancing your luck you could import them from advanced generation games. And, if thats not an option you may chance your luck here at the trade sub section of this forum.(conviently located above this page)
Hope that helped.