There are many different ways of finding a hypnotherapist. One of the best is to ask around - friends, family or work colleagues may have had a positive experience and be able to recommend someone.
An alternative way of finding a hypnotherapist is through your GP. With GPs becoming more open to alternative therapies and treatments, it's quite possible that your GP will be able to recommend someone locally.
Other ways of finding a hypnotherapist would be the phone book or the internet - they share similar upsides and downsides.
The advantage of both is that you are likely to have a choice of practitioners - probably too many if you search online! However, the downside of both is that without any form of recommendation, it will be hard to know if one is suitable for you.
The first consideration when choosing a therapist is going to be their training and qualifications. Unfortunately this is a field where there are literally dozens of different organizations offering certification, so unless you have a knowledge of what you are looking for it's probably best to go with one of the bigger national governing bodies such as the British Association of Counselling and Psychotherapy (BACP) or the British Association of Medical Hypnosis (BAMH).
Once you have picked a therapist, call to make an appointment. This is a chance for you to ask questions and be sure that they are offering the type of treatment you are interested in. Be sure also to ask about fees, as these can vary widely.
Finally, it is worth asking about an introductory consultation - many therapists offer this as a way for you to see if this form of treatment is suitable for you.