This can depend on where you are based (UK or US or somewhere in between).
So here is my list (in no particular order).
- Zorb (not easy to get outside of the US, but super absorbent, light and quick drying)
- U-Haul blankets (again a US thing, and I've personally never used them but have heard good reports from others)
- Mattress protectors (readily available everywhere and pretty cheap - you are looking for 100% cotton ones - not polyester)
- Old bath/beach towels (readily available and although they make for a slightly heavier liner they are good becasue most people already have them on hand)
I would also say that it depends on the size of the cage liner - I use zorb for my big 5 x 2 liners, and cut up beach towels for all other sizes.