Annette, many colleges and universities offer professional training. We are located near Penn State and have used their training and development services for primarily front line supervisor training but they certainly discuss this issues of working across the multiple generations in the work place. Perhaps you could try connecting with some universities in your area that may have a robust training and development department.
I would have to say though that I don't see much call for workplace behavior training to be very different for millennials than any other generation. As long as the focus is on what's acceptable rather than "don't do these things" since the mind isn't going to hear the "don't" in that phrase anyway. Hope this helps!