BMI is just a formula that you can do yourself. I am no good at math, so I use a calculator on the web.

http://nhlbisupport.com/bmi/
However I think what your talking about is body fat percentage. No idea where to get the water tank method done, it's a frequently asked question and no one ever seems to know the answer

I would suggest asking your doctor, I presume large hospitals would have the facility.

I get mine done at my gym by electrical impedence, where you lie down and the stick an electrode on your foot and one on your hand. My trainer is convinced that for me it's completely wrong - far too high, I on the other hand, don't feel this. When I started I was 51.7% body fat, and now I'm 41.5%.