Relay kit? Not sure what you're looking for, as most aftermarket horns come with a relay. All you need to add is wire from the battery (or unswitched post of the starter relay) to the horn and the other connection on the horn to a good ground (not to the battery).
The Fiamms and a couple of other aftermarket horns are sold without a relay. Making the two connections as you described will toot the horn as long as you keep the wires connected. There is no switch in your circuit.
I used a generic auto (12 volt, 20 amp)relay and fastened it to the frame strut the old horn was connected to. My new Fiamms were mounted higher inside the frame piece just inboard of the mirror mount. Your wire from the battery (new fusebox in my case) goes to the relay power input contact, the power output went to the horn(s).
Horns are grounded and the relay's coil got the old wire from the horn button. Second relay coil terminal was grounded. I admit this might not be too clever. When the relay fails, I will have to remove the tupperware to get to it.