There is no "factory installed one", so it's easy to get installed.

You can buy both the OEM DVD system (Rosen) and the install kit from someone that sells Land Rover parts, including your dealer.
YIP500350: LR3 DVD system (Rosen)
VUB503142: LR3 Installation Kit
You can attempt to install it yourself, or hire Land Rover or a shop to do it.
FYI: It's likely you could get two larger screen DVD systems, one installed on each side, for less money than the cost of these parts and the labor a Land Rover dealer would charge to install.