Hi Paul,
I don't think repro horn buttons have been made recently although Harrisons (
http://www.workin4u.com/earlyspares/) were experimenting with re-conditioning FE bonnet badges (a similar item)...might be worth talking to them.
There is some discussion on this site of producing repros of the black things (escutcheon plates)...see
http://www.fefcholden.org.au/cgi-bin/yabb/YaBB.cgi?board=resto;action=display;num=1059393463;start=8not sure what has become of this? They also pop up quite often on ebay. If you don't have any luck with the above I have a spare pair in pretty good nick. If you send me a PM with your e-mail address I can send you a photo and you can see if they are what you are looking for...
cheers, jon