Type in "recommend screen protector 6d". It will lead you to the 6d users thread, where people have been discussing their preferred screen protectors, etc
Buy from eBay! I got mine from there... I'm not particularly concerned about how much light it can let through though, just want something to protect the LCD screen...
Necessary? Up to you. If you are careful enough or not too kiasu over some resale value you can skip it. But it doesn't hurt to put it on, since it's not involved in the picture taking.