There's an achievement called Stay Classy Steelport which reads as follows:
Kill 25 Gang Members each with 'the Penetrator' AND the Fart in a Jar.
I've had The Penetrator for a long time, but I've completed Act 2 and I've still seen no sign of the Fart in a Jar. Where do I find this weapon? Is it a mission reward? I haven't seen it as a weapon to purchase in Friendly Fire either.