What kind of bedding and litter is the best to buy for a pet rat?

Aspen shavings are the best and are found by the name Sani-Chips at most feed stores and lab supply stores. Newspaper bedding is not good for rats because the colored inks can be toxic, sodo not use newspapers. Corncob is okay, but not very absorbent or nice to sleep on. Corncob bedding also has a tendency to grow molds very quickly. Pine shavings have been known to carry red mites, cause liver and kidney damage, and respiratory disease.Heavily soiled litter should be scooped out daily, and more litter added if needed. The whole cage should be cleaned and new liter and bedding provided once a week or so. 

My favorite bedding for small animals is Carefresh, the odor control is great, it comes in a variety of colors, I just get the non colored type as it is cheaper. 
Your favorite pet bedding in eye-catching, non-toxic, colorfast hues.The ideal litter for small animals such as guinea pigs, hamsters, rats,chinchillas, rabbits, reptiles, and hand-fed baby birds.

Aspen Supreme Pellets use a special blend of fresh aspen wood particles--a natural by-product of a renewable resource. Contains no cedar or pine, which can be irritating to small pets.