According to the Feingold Association of the United States website, accessed on 2 July 2013,
“The majority of the foods can come from your neighborhood supermarket. You might have to go to a health food store to get natural toothpaste and natural lollipops, or if you prefer, they can be mail-ordered from suppliers who cater to Feingold families.”
There are no formal qualifications for people using the Feingold diet for since any individual, parent or carer can choose to implement the diet.
However, if you are contemplating 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 dieticians in the UK.