This is the officially licensed Ghostbusters Slimer Halloween mask designed by Rubies Costume Company and sold by a ton of places online (just do a Google search). It sells for around $28 and meets all my criteria for a good Halloween costume: It's recognizable, and easy to drink in. You should get a couple friends to go all out with actual Ghostbuster costumes, then you can just wear this and photobomb all their photos. PROTIP: Try to get yourself to puke right before the flash goes off.
Thanks again to E V I L A R E S, who's so evil he always Halloweens as the devil and pokes holes in people's buttcheeks with his trident.