Do a search on Google.

Pet stores and garden shops have it.

You can also find it on e-bay. Seems kind of expensive.
mbetts

I got a 3 brick pack for 6.50 at the pet shop. Each brick expands to 8 liters.

Not free but reasonable for what i use it for. I sit a whole brick into the drip pan and let it soak up the lechett drips. Then i scoop out the soft crumbled stuff and put it into the top bin where it magically becomes bedding. I'm still on the first brick its lasted over 2 months now and it still looks something like a brick.