They weren't stock, and for the short blasts that you'd use the horn for, you really don't need a relay, but you will need to use at least [does math] 14 gauge wire. you'd probably be better off with 12, actually, but it would work okay with 14.
6v Horn Relays (or any 6v relay, for that matter) are difficult to find and those that you DO find cost an arm and a leg.
If you were to put one on, you would probably mount it to the firewall. Wire the button with 14 gauge and wire to the horn and to the main power point with 12 gauge.
There should be instructions that come with the relay for the particulars.
