Zumar Industries is a major supplier of Traffic Signs. They do custom order stuff as well. They have a plant in Phoenix as well as Santa Fe Springs Calif.
and do have to-the-trade pricing which is not bad.
Easternmetalsupply.com
My guess is ordering the blanks from them locally and then getting vinyl applied locally would be the cheapest method of mass production?
Please come back and let us know what the result of your research and bid were. From my minor experience with such bids in the past, I found that anyone capable of producing and supplying signs of that type and quantity were usually bidding themselves...and at rates that you could not think about competing with.