If you want a realistic-looking mask in the likeness of any person in the world, your best bet is probably to hire Hyperflesh, the outfit that creates those amazing Baby Masks. Not in your budget? Join the club. For the rest of us, I have a feeling we'll do well enough wearing these Freak Masks instead.
Made by Firebox, it's utterly simple. Just a lycra hood that covers you entire face with cutouts around the eyes. That's it. The trick is in the high-res personalized print, which makes it look like you tore off someone's face and wore it over your own, Hannibal-style. Seriously, it's creepy.
These Freak Masks should work great for anything: Halloween, cosplay, robbing banks, creeping out mom, stripping your clothes on Snapchat while wearing a Miley Cyrus face, and anything else in between. To get one, simply upload a high-res photo of yourself or someone whose mug you'd like to wear over at the product page (minimum 500KB) and order it for printing. Firebox recommends using a high-res solo shot where the face is clearly in focus and forward-facing, with a plain background. The balaclava can maintain the integrity of the print for over 100 washes (considering you never wash your headwear, it will probably last forever -- albeit, in smelly fashion). Oh yeah, the material is stretchy, so it's one size fits all.
Firebox has the Freak Masks available now, priced at £24.95.