I'm in Australia and had it done at a laboratory here. You send in a sample of your hair (about 1 TBSPN from the nape of your neck - uncoloured or treated of course). The hair is analysed for mineral/toxic elements - an indication of what's in your tissues. It is said to be very accurate and a good way to find out about mercury levels etc.
Do a search on google and see what you can find. I don't know of any good sites.