Hit up member SV&HOdan. he has good prices, fast shipping (from out of Pittsburgh area) etc for used parts. First go to for used parts when I need something. SHOsource is also an excellent vendor to support these cars.
Are you wanting a good used Ford DIS or just a working unit? I have to go out to my shop today to look for something else and I know that I have good Ford units, but you can also buy brand new aftermarket ones on Rockauto if that's all you need.