The Penetrator can be bought at any Friendly Fire store. It is a melee weapon and looks like a giant purple dildo. This weapon is also awarded to you during the story missions. The Fart in a Jar is obtained by going into a Friendly Fire store and upgrading the Flashbang to its max level.
You have to kill 25 gang members with the Penetrator and disorient 25 with the Fart Jars. To quickly do this you can toss one Fart Jar at an enemy and then kill them with the Penetrator. To simplify things, kill the gang members while standing in front of a Friendly Fire so you can quickly restock on your Fart Jars and remove the notoriety if things start to turn ugly.