Where I buy from (a smoke shop), there's a sign that says you have to be 18 to enter. I guess they expect you to be honest, but don't really care, as long as they get business. Apparently I look old enough, because I've never been IDed, and I'm 17.
Some herbal supplement stores sell it, too. But I'm basing this on Portland, OR, and we're all kinda weird here.
The kind they sell at the store I go to is "Purple Sticky Salvia", and they have a web site at purplestickysalvia.com
I am by far NOT an expert, though. Just my feedback.