I did a henna course with an independant tutor, but my insurance company wouldn't accept it.
As Ruth says, do not buy black, it is dangerous and can cause permanent scarring.
colleges sometimes offer it as a short CPD course.
Sorry can't help, and wouldn't recommend you have my tutor, he worked abroad doing these henna tattoos at holiday resorts, so obviously knew what he was doing, but he was a **** teacher. And I know I should have checked with my insurance company first, but really wanted to do the course.
Don't forget that if you are working on children you will need a current CRB check and parental permission.