I tried buying the "Ferrari" horns from RM. They had none in stock and they referred me to a mail-order house specializing in Ferrari parts. THAT place told me the Ferrari parts are the exact same parts as the Hella kit, which they sold me. BTW, the Hella kits generally run $70-80.
The NSX has three horns - two regular ones and a third in a separate location (I think the latter works for the alarm as well as the regular horn). I replaced the two with the Hellas, but left the third stock one in. Sounds great.