Try googling melatonin, there seems to be a couple of websites you can get it from. (sorry my computer wont let me do links)
Alternatively have you tried shops like Holland & Barrett to see if they stock it.
Thanks for your concern Bathsheba, but that is why I want the synthetic melatonin, which is almost identical to the body's own naturally produced hormone, and non-toxic. The potentially harmful types of melatonin are derived from animal material (often bovine) which can carry disease and infection. I wouldn't touch that stuff with a bargepole.
A good place where you can find these is https://www.vitaminexpress.org/uk/melatonin-supplements. I buy my melatonin from them because they often have good deals and their quality has been fine. The delivery doesn't take very long either.