You can buy them in boots but they ask loads of questions apparently!!!
Your CDC will probably have them to sell.
I personally dont use them, some people swear by them but I have never bothered, I know as long as I do this diet right, chug at the water (4 ltrs min per day!) then this diet works wether I show ketosis or not (3s 4lbs of loss shows I do someting right!!)
Dont panic about them if you dont have them, you can speak to your CDC to get some if you would feel happier knowing, for some it gives a boost to see them in the pink.