Sawdust should never be used for rats are this has cause resp issues. Some good bedding you can use would be littlemax, bedmax, aubiose, finacard, ecobed, hemcore, Megazorb, Greenmile and Flaxcore.

Most Horse bedding is suitable. I’m currently using Bedmax
I also use Walmsley cardboard squares I get from a local farm in a bale or Aubiose which is a hemp bedding. For a small group of rats this could last you nearly 3 months so if you have room to store it I would get it a bale at a time.

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.

Various other pet store sell bedding in smaller packs one being Rat Rations who do a great many wonderful things for your rats also recommend Rat Warehouse.

Rats also need other beds such as hammocks

Some good hammock and bed makers