[Meneltarmacil]

The problem with getting supplies from Sauron seems to be mainly a geographical one. The most direct route between Mordor and Isengard would be through Rohan, which Saruman was unable to ever fully control, though he did try.
I suppose Sauron could have sent supplies via ship from Umbar around Gondor to the coast of Dunland, possibly even up the Isen (though this is unlikely since Rohan controlled part of that area). However, this theory too runs into problems as Gondor probably wouldn't allow the Corsairs anywhere near their waters.

Of course, there is one other source of supplies that we know Saruman had been in contact with. He could have been secretly receiving food supplies and possibly some basic raw materials from the Shire (specifically the Southfarthing), though the distance is a problem here.

Is it possible that Saruman had set up some kind of large-scale food production system in Dunland? He certainly seems to have had a lot of influence there. Metal and wood are not much of a problem, since he had Fangorn's trees and the minerals present in the Misty Mountains.