I bought some over the counter in the US while there on a training course. I was having real trouble sleeping (except in the classroom and sim) so in desperation decided to give it a try.
Recommended dose was "..one to three tablets.." which wasn't very helpful. I started with one then upped the dose the next night to two. Not sure if it made any difference. I still couldn't sleep and not knowing exactly what it was doing to me I threw the bottle away.
If your long haul flights take you that way you could just buy some.