In looking for monitors I found this site. Haven't ordered anything myself, yet, but they have some pretty decent prices.

Make sure you look at the specs on the monitors, and such, before you order. Some are lower models with lower resolution. But I know a lot of people aren't to worried about that for things like headrest monitors, visor monitors and so on.

If anyone has dealt with them before, feel free to chime in, I just ran across it a few days ago and figured I'd share.