Sawdust should never be used for rats are this has cause resp issues. Some good bedding you can use would be finacard, ecobed, hemcore, Megazorb, Greenmile and Flaxcore.
Most Horse bedding is suitable. I currently use either Walmsley
I do not recommend fleece as a main bedding apart from to make hammocks.
This is due to the fact rats need to forage and they are unable to do this with fleece. Fleece gets very smelly very quickly and rats with rip it up and hide under it.
I also use litter in litter trays I use back 2 nature but most paper based litter is fine. Litter training rats can make it easier to keep the cage clean and smell down as well.
Rats also need other beds such as hammocks
Some good hammock and bed makers