This used to be the realm of car stereo shops, but even Best Buy and Circuit City can install these systems for you. Cost depends on how many screens you want, how big you want them to be, style (drop from the roof vs. headrest mounting), if you want vhs or dvd, and if you want the sound piped through your stereo or you want to use headphones (which come wired and wireless). You can even go really cheap and get the Rampage bag setup which is essentially a vcr with a built in screen that hangs in between the front seats and plugs into the cigarette lighter.
I'd suggest that you post in the car audio forums and maybe check out some of the many websites/forums that talk about this topic at length. From my point of view it sounds like a single drop down screen and some headphones would be great for what you want (unless you want to listen to barney all the way across the country).
-Jade
