The supply and availability of the ingredients required for special diets varies enormously between different diets and sometimes between different countries. For example, obtaining gluten-free or casein-free ingredients can be difficult in some countries.
The quality of special diets varies enormously depending on the diet, the ingredients used, and how rigorously you follow the diet. Most specialist diets are unregulated and uncontrolled, since anyone can choose to follow them.
Various support groups exist which may be able to advise you on how to follow specific diets and there are usually resources (such as books or DVDs) which may also instruct you on how to follow those diets.
However, if you are contemplating following a special diet, you should seek advice from a paediatrician, GP or dietitian. The British Dietetic Association has information about qualified dietitians in the UK.
There are no formal, internationally validated registered qualifications for people using most special diets for individuals on the autism spectrum, since any individual, parent or carer can choose to implement the diets.
However, if you are contemplating such a significant change of diet you should definitely seek advice from a qualified medical practitioner, paediatrician and/or dietitian.
The British Dietetic Association has information about qualified dietitians in the UK.