Big John,
This ebay seller sells ready to fit horn nocks from time to time. Recently they sold a nice pair of albino horn nocks:
http://cgi.ebay.co.uk/ws/eBayISAPI.d...MEWA%3AIT&rd=1
If you like the look - you could contact the seller - I think they are going to be ordering some more.
In terms of sourcing unusual horn - might be worth checking this site out:
http://www.traditionalmaterials.co.u...odhornbone.php
Hope it helps,
Dave