I get it from Zooplus, it was actually a thread Mark started that made me try it. It is quite cheap too. One small downside is if you don't get to the wee clumps quick enough and they dig through it, it does break up a bit but that is the only downside so far.
I do worry about blocking the toilet, particularly as my landlord is already not thrilled at the presence of cats but, well, it is so much easier than running down to the bin with a little bit of waste in the peeing rain. Yes, I am lazy.