Many people probably didn't know about this so I thought I'd share this very easy tutorial.
I noticed that you cannot buy these masks which is very disappointing because they'd be very awesome in online but in the meantime I discovered a trick to wear them without wearing the stealth outfit (Like wearing the police outfit without the helmet trick)
Basically you need to complete the main story first, obviously.
This won't work on most types of clothes except suits (which looks badass)
1.Pick Michael or Trevor
2.In your wardrobe under outfits pick "Stealth"
3.Then go to "Suits" and go to "Suit Jackets" and pick a jacket (Jacket, not a coat or the mask disappears)
4.Find some matching pants under "Suit Pants"
5.Boom, you now look alot better than before