Henna question

[riath]

I have decided that I want to try using henna on my hair, but I figured it was better ask advice before plowing ahead without knowing what I'm doing. My hair is currently about tailbone length, brown, straight and somewhat thick. I did read the FAQs on henna here, but my first question, where in the UK is a good place to get proper henna? I love my hair dearly and would hate to end up with something that would make it fall out! I have previously used commercial dyes on it, the last time being around 3 or 4 years ago. Second question, how much would I need to use and how long should I keep it on when I do come to henna my hair?

Thanks in advance for helping out a henna newbie!

Also, hair toy related question: Any good UK sites where I can get hair sticks?