An electric horn circuit consists of very few parts
Fuse----Relay contacts----Horn---Ground (Suspect #1: relay 2: Ground)
Ground---Button----Relay coil (This ground is NOT suspect)
Fuses.. TEST with test lamp.. To do this connect clip of test lamp (Link below) to any good ground.. Toch both metal contacts (one on each end of fuse: LIGHT LIGHT = GOOD DARk DARK = no power or bad ground clip connection LIGHT DARK or DARK LIGHT = blown fuse.. Some times a volt meter will show good when the light shows bad.
What spares to carry? I carry an assortment.. Sold in stores like Pep Boys this has several fuses of several sizes.. Now physically blade fuses come in at least 3 sizs.. Mini-Standard-Jumbo (Large) I have a couple of the mini and jumbo (ones I am most likely to need) and the assortment is standard since most (NOT ALL) of my fuses are that size.
Circuit Tester 6 12V Volt Electrical Automotive Test Light Probe Auto Tool | eBay