You can obtain these from many Army surplus stores in the UK. I bought 4 in 2004 and have used them for drinking water, boiled first of course, they were £18 each then? I gave 2 to a friend as a gift, still using the other 2 though, if you are in a warm climate strap one to the roof through the day and when you stop hang it in a tree and take a shower, lovely.
I steralise mine with a standard water container cleaner occasionally. otherwise, as already mentioned, the water becomes smelly and tastes bad.
I realise the Pedro is in Spain, the UK comment is for the Brits amoung us.