I actually just looked this up, and it's only $30 bucks. 
https://www.amazon.com/LeeKooLuu-Monitor-Waterproof-Fulltime-Optional/dp/B06XKPQ6YZ/ref=sr_1_3?s=car&ie=UTF8&qid=1512925167&sr=1-3&keywords=back+up+car+camera
The reviews said there is a wireless option one, but you have to hook up a battery supply, otherwise they run this one into the car on their own. Also, it has 4.6 out of 5 stars with over 100 reviews. Hope this helps you. Good luck!
If you meant specifically one in the mirror, I briefly saw one on Amazon, but it was $110 bucks so I figured the $30 one would be a better option, but that's just me. :P
