Decide what you want. A monitor, built-in DVD player, game kit, you name it.
Look for manufactures that create retrofit headrests for specific vehicles. These headrests are specific to each vehicle type. Sites like Park My Auto and eDirectory have a wide selection of these (see Resources below).
Go to Mobile Video Zone. They make an "active matrix wide screen TFT LCD headrest monitor with universal leather headrest". As the description reads: "Simply Drop In & Connect To The DVD, VHS or Playstation2. Hard Wire, 12V Power & You're Done."
Check eBay. You can find lots of retrofit headrests there, with lots of DVD kits, monitors and all.
Explore Shop Wiki. They have universal retrofit headrests, as well as specific model ones (see Resources below).
Head on down to the big box stores in your area. They often sell portable DVD players with headrest mounts.
Visit a shop that specializes in car audio systems. Shops like those will be able to be as creative as your imagination (and theirs) and create a system for your car that will blow you away. They will have access to many retrofit headrests.So you've got a cool car and you want to make it cooler. One simple way is to retrofit your head support-though the head support can be anything but simple. Today's headrests have cool options like DVD players, twin DVD players and more. Read this eHow to learn more.