I just finished reading Area 51, An Uncensored History of America's Top Secret Military Base, by Annie Jacobsen, and after reading this book, I'll never again believe in UFO's and aliens. This book dispelled those ideas.
I doubt very much that you can find any proof that real aliens exists, as you are entering CIA territory, and it would be impossible to get any information on that subject from the CIA.
Not even Presidents have been able to get information about Area 51 here in NV from the CIA!
And, probably best it all remains a secret.
One day, many decades down the road, perhaps some of that info will be declassified. Maybe, maybe not!
The UFO craze of the 50's/60's was a result of the CIA flying those spy planes (A-12/Oxcart) around the world, and the CIA couldn't very well tell the public what they were up to, as they were used to spy on China/Russia. Even commercial airline pilots would see these alien looking planes and conclude they were UFO's, and the CIA just got a good laugh out of it!