[A_Brandybuck]

I think, that Saruman's economic infrastructure is based on the production, which Isengart naturally have. I think, that Isengart itself must have a nearly autarc system or the possibility of being nearly autarc.
In the time before the coming of the Rohirrim, Isengart was an outpost in the nearly deserted fields of Calenardhon. The people living in Isengart must have fed themselven alone (I suppose).
That would mean, that there was an existing infrastructure on which Saruman could build his own economic infrastructure.
We know from Saruman, that he is a cunning head and we know also, that Saruman have some economic skills, because of the story with the tobacco and the Shire.
So, it is likely, that he did some trade not only with the pipeweed in the Shire, but also in other things. He had the mountains and the Dunlendings directly in front of his home. Consequently there was a potential of trade there.
We know also, that Saruman had many activities in the region of Dunlendings. Dunlendings were in his army and they succumb to the propaganda of Saruman (thinking that the Rohirrim are very cruel to his prisoners of war). I think, that such a propaganda needs some time and trust on the side of the Dunlendings towards Saruman. This time and this trust could Saruman had gained through economic things.

Which I can't explain with this theory is, how Saruman could feed thousands of orcs. But I find the Dol Guldor theory very good. ;-)