As well as referring to the sites listed below, it would also be worthwhile looking at Training Schemes to see if there are any in your area or if the organisers can provide you with any advice. In addition, many Local Organ Associations have teachers listed on their pages. You could also approach the organist in your local church, cathedral or school who would also be able to provide advice.
Incorporated Society of Musicians
Mentoring opportunities through Oundle for Organists
Online learning through RCO (membership required)
The Organ Manual 