You don't need to buy it done mate, it's extremely simple.I'd deffo give it a try, sounds like a winner, unfortunately they don't sell it in the shop by ours.
Recipe is:
Get some yoghurt (without any flavouring (!), to my best experience and knowledge so far - Tesco's Natural Greek Yoghurt is the closest to the one I have back home in Bulgaria)
Put 2 tablespoons of it in a glass (300 or whatever ml). (For bigger container - more, ofc)
Put water and stir until it's drinkable.
Put salt in it, to your own taste. Of course for hangovers this changes to "put as much as you can without your face turning into a fist from too much salt-taste")
Enjoy.