I got mine through word of mouth too. It took me a while to find someone though. There are a list of BE coaches on the BE website. I would also echo the above comments.. perhaps get involved in a local riding club and try a few clinics sounding out the instructors they use?
Good luck
Al's was recommended to her by someone who knows her very well, and knew she needed some help from someone! Best instructor, everyone who's taught by her raves about her and she has amazing comp results too- what more could you want?!
Word of mouth and instructors already visiting the yard are usually the most popular. Otherwise you could try tack shop notice boards, google (there are an increasing number of instructors with websites and advertising on the web), the BHS register or your local pony club.