I generally found them in Northern Greece in my Greek Campaign. They are kidn of all over the place there as I remember recently hiring the following Mercs in Ionia (right near Sardis): 2 Bastarnae's, 2 Cretian Archers, 2 Rhodesian Slingers, 1 Merc Hoplite. A nice assortment I have to say.

So I'd say they can be found anywhere from Northern Greece to Western Anatolia. They're amazing troops.