We use pine pellets – made from surplus wood waste (pressed sawdust). Once the pellets get wet they turn to saw dust. If you sift it, you can use them again – there is very little waste. Sifted saw dust can be used in the garden for mulching. The best litter tray I’ve found is Oz-pet’s sieve tray. Since I’ve started using them, I have significantly reduced the amount of pellets I use.
Check Oz-pet website if you need more info about the tray: http://www.oz-pet.net.au/index.html
In New Zealand you can get the pine pellets at your local Bunnings. It is called Azwood and it is sold as fuel for special pellet fires. It is the same product that is sold in smaller bags in pet shops, for a much higher price.
You can also get them from: http://naturesflame.co.nz.