usually you would buy henna powder (I've found it in some grocery stores) and mix it to make a paste, which you use to draw your own designs on.

Also, I've had henna tattoos last a few days to a few weeks, depending on where they are and how well you look after them. If you get one done, they usually say it will last 6-8 weeks, but my experince is 2-3 is the longest that it wil actually look good for.