Just for info, Oko (which Dawn recommend) and Cats Best (which Mymblesdaughter recommend) are the same thing - its called Oko Cats Best and I use it too! On the face of it it might seem really expensive but it lasts so long it actually works out really good value. It's flushable too but I only flush poos and just use nappy sacks to bag the wees. You need to have it nice and deep and you just top the level up every so often. The only negative, as MD said, is the tracking but I keep a carpet sweeper in the room with the trays, plus i have hooded trays and litter tray mats which helps keep on top of it instead of hoovering daily.
I used to use chick crumb (chicken feed believe it or not!), it's very similar to Oko cats best and also worlds best as they're all corn based hence being flushable. I used to get a 20kg sack for £8 from Jollyes but I bet it's gone up by now.