Melatonin is widely available within Europe and the USA but restrictions apply in some countries, such as Australia and the UK. According to the NICE website, accessed on 12 September 2017,
“Only 1 form of melatonin (prolonged-release tablets) is currently licensed in the UK for the short-term treatment of primary insomnia, characterised by poor quality of sleep, in adults who are aged 55 years or over. Additional melatonin products are available from special-order manufacturers or specialist importing companies, or can be purchased directly online.”
You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy.
In the United States, melatonin is sold without a prescription in chemists and health food stores. Therefore some providers will have professional credentials and qualifications, while others will not. In the United Kingdom it can only be obtained by means of a private or National Health Service prescription, supplied by a suitably qualified and experienced medical practitioner.