The "stoneheads" are in the southeastern part near Syria. In Istanbul along the blue line streetcars there are travel companies that conduct tours. You'll find several near Sultanahmet Square. I picked up a brochure last week in Istanbul: walkabout.com.tr., Walkabout Travel, Alemdar Cadessi, No. 3/B Sultanahmet, Istanbul.

Note: I have no experience with this company and they're not the only ones that could give you a tour.

Can experienced people chime in here about touring Anatolia? I'd like to do Ist. and Anatolia (including the heads) on my next visit. Istanbul was awesome.