You can get it at Pets at Home and Jollyes - and several places online aswell. I'm a recent convert and thought it was going to work out too expensive to begin with but a 40L sack lasts two cats, who very rarely go to the toilet outside, over 3 months
It has one big downside in that it's terrible for tracking/trailing but the plus side is that there are no smells whatsoever, it lasts an absolute age and the boys like it