I presume the nut changing (hehe) is for electric guitar? Well davies does this, but what nut would you like? Cashew or Macadamia? In seriousness there are various kinds of guitar nuts like Buckethead, Eric Johnson....
REALLY! You could have brass, bone, wood based, graphite, stone or the usual plastic based ones. They give different tone characteristics as well as a difference in string protection. Also note if yours is a Floyed Rose the nuts on offer may differ from what you need.
In addition to Davies there is Guitar Workshop at Bras Brasah, Zen Guitarworks, Luthermusic, Standard Value, Look for Beez... Blackwood could help you out too.