I don’t know if buying a green screen is the first thing that comes to mind when developing an e-learning dvd. You need to decide what kind of DVD are you going to make. Is it for playback on any set top DVD player or only for playback on PCs? What about mac? Your decision would dictate the approach.
If you choose the set-top DVD route, then any advanced DVD author could help you out. It will likely require some advanced scripting knowledge.
On the other hand, for PC playback, a flash developer might be best. You may have one of them laying around.