Straight off the bat this is going to determine what you will end up with, Cameras can be had for anywhere between $50 all the way up to $500+ depending on features. Set yourself a limit for spending and shop around. Be aware that cheap and cheerful is not always a good idea when it comes to a camera. Also be sure to do your homework on sellers to get the best deal, Some cameras can fluctuate between $150 on price from others.
Many cameras have a secondary rear facing camera that will show the car cabin, you may question the use of this feature but be aware this can save you a fine in instances of being accused of being on the phone while driving etc, They will also give you some footage in the case of a rear end impact. Other models will have a camera connected by cable which you can place on your rear window, which will help cover you if anything happens behind you