Those little prongs break easy.
The dealership where I lived told me I had to purchase the whole horn button including the wires that went into the steering wheel, for something like $75+ at the time if I remember correctly.
In my case, I was able to locate the spring and the broken prong. I used some super glue and it has been fine for the past two years. Then again, I don't use the horn very often.
I was pointed to Dali Racing as a place to get them. Here is the link for a left horn button.
http://daliracing.com/v666-5/catalog/index_browse_part.cfm?focus=1831
It is listed at $33 + $6 S&H.
I've never ordered from Dali Racing. I have read on prime where some members have had issues with them and others haven't.
There are also other Acura NSX Parts dealers listed on this site, who seem to offer much better prices than just walking in your local Acura dealer (or at least in my case).
I myself have been meaning to purchase two new horn buttons, to have around as spares. Since my right button did break and is currently working okay with my repair, but I have a hunch it will break again. And it seems that the buttons have a tendency to break.
Good luck and keep us posted on where you end up getting a replacement and the cost. I might just decide to get around to ordering a spare pair myself.
- Rod