Indeed, probably most chinese herbal shops or Amazon, one of my partners is an acupuncturist so I have a more direct source, but the internet seems to provide, and it's quite a common OTC remedy in China (and equivalently, in various Chinatowns.)

One thing I seem to remember when looking into what was going on with it a few years back is that the Anglicized spellings vary all over the place, which which can slow things quite a bit. While that's listed as 2-3 pills 2-3x/day, I find an effective dose to be as little as 1/2 pill once for me. As with melatonin, I'll usually take it and then wait 15 minutes before attempting sleep again.