paper pellets are probably the safest bedding you can use for your rats, I would completely stay away from any wood based bedding its not safe for the rats. one of the easiest and cheapest bedding is just shredded newspaper, it smells bad after a while but the ink is safe and its cheap. I personal use cat litter, yep in the whole cage i buy it at sams club any one that is not special just a regular clay litter works amazingly.
Aspen is the best wood bedding to use for rodents. It doesn't have the irritating oils like pine and cedar. There is also a good paper bedding called Carefresh. It looks like chopped up egg cartons.
carefresh is what i used when i had rats, i put down a layer of newspaper and then a layer of carefresh. it worked wonderfully for me. i avoided wood because of the respiratory issues along with they could stab eyes or something (i was always paranoid). i give carefresh a 10 out of 10, it even comes in different colors!!! lol, a bit more pricey than wood chips but still worth it
I'm not sure what it's called, but for small rodents you can use those nice little fluffy, papery, cottony things. And they have different colors if you wanna get creative. Haha, I just wish I could think of the name! I know you can use them for like any kind of rodents--I used to have it for my gerbils and hamsters.
Got carefresh! thanks for advice. it cost me 21 to get 60 liters, i spent it on my birthday money. xDDDDDDDD! but it was worth for my Jebby. She seems happy. I got her crickets, yoguats drops treat and timothy hays. plus i pay one bag of hay and got free! I think i probably spent 30 on this for Jebby and Jackie(leopard gecko)'s calcuim worms. :3