303abuser it’s otc and really cheap, any pharmacy should carry it.
I use it for jetlag. You can get it from places like Holland and Barrett, you don’t even need to find a proper pharmacy.
A lot of airports sell it in the travel shop-type place, that sells all the junk like overpriced travel adapters, neck pillows and playing cards.
EDIT: You now cannot get it in Holland & Barrett, I just checked that. It has been reclassified in the UK and now requires a prescription. I have a massive bottle of 180 caps that I bought in Shanghai airport. Valerian or Ashwagandha will do pretty much the same, and is available in Holland & Barrett.