It is found in Camp Searchlight's fire station. Once the key is obtained from Logan, head upstairs and search the bathroom stalls, look in the right side corner of the stall with the skeleton to find Knock-Knock. However, it is in very poor condition and requires repair.
Logan can be killed or the quest, Wheel of Fortune can be completed to obtain the key.
While the standard fire axe is a pick-head axe (also traditionally called a fireman's axe) intended for use in structure fires, Knock-Knock appears to be a Pulaski tool. The Pulaski (named after its inventor, legendary US Forest Service firefighter Ed Pulaski) is a wildland firefighting hand tool that combines an axe and an adze. It is an extremely versatile tool that can cut, scrape, grub, or dig.