First and foremost, it's important to note that clay cat litters should not be used for ferrets. Clay is incredibly dusty, even the low dust varieties. Plus, that stuff smells so bad when wet! (really, I don't get why people use it. Blech!)
The same goes for shavings. The dust and oils release can cause respiratory issues. Plus, they make a huge mess and aren't worth the trouble.
So, what litters CAN be used for ferrets? See the list below, with the pros and cons of each.
These are a popular choice with many ferret owners. Wood pellets are made with various types of wood sawdust, and compressed into pellets. They provide decent odor control and are usually widely available. They do break down into dust when wet, so scooping is a bit more difficult, and requires completely emptying of the litter tray. If you have a ferret sensitive to dust, use caution with these litters.
Wood pellets may be sold as horse bedding (check farm stores such as Tractor Supply), or as wood stove fuel. If buying pellets labeled as fuel, ensure that there are no added accelerants.
They are usually very economical, and cost around $5 for a 40 pound bag.
Paper litters are made from recycled newspaper and compressed into pellet form. They provide fairly good odor control, and do not break down when wet. This means it is fairly easy to remove soiled litter.
Paper pellets may be sold specifically for ferrets (such as Marshall ferret litter), but cat-specific brands are identical and usually cheaper.
Cat-specific brands include Yesterdays News, Fresh News, and Exquisicat. I have not notice a huge difference between brands, except that Exquisicat has larger, harder pellets, Yesterdays News tends to break down easier, and Fresh News has longer pellets. Other than those small differences, I feel they perform equally well.
Paper pellets cost more than wood pellets, ranging $10-$15 for a 20-30lb bag, however, they are an excellent low dust option for more sensitive ferrets.
Walnut/Wheat/ Corn Litters
Recently, vegetable based litters have made an appearance on the market. They are made from items such as walnut shells, wheat, and ground corn. They offer a safe, low-dust alternative to traditional clay cat litters. They have a small particle size that is similar in texture to traditional cat litter.
I am not aware of any vegetable litters that are marketed specifically for ferrets, but popular cat-specific brands include Blue Buffalo walnut litter, Scwheat Scoop, and World's Best corn litter.
The generally have very good odor control. Most vegetable litters are clumping, so keep this in mind if you have a ferret who snorkels in litter. I have not had any issues with these litters clumping to ferrets' private parts, possibly due to the larger granule size (when compared to clay)
They are very low dust, and pose a very minimal inhalation risk.
These litters tend to be more expensive than pellets. World's Best corn litter runs around $10 for a 7lb bag. That said, they do last quite a while since they can be sifted.
Clay litters are also known as traditional cat litters. They are made from clay, and are grey and granular in appearance. Even low dust varieties are incredibly dusty, and clay dust can build up in the lungs. Because of the clumping nature, they would not be a good option for ferrets who snorkel in litter. These litters also pose a blockage risk, should a ferret ingest the litter.
I personally find the odor control of clay terrible, and the wet clay gives off a very unpleasant smell.
Because of the cons associated with clay litters, they are not one I'd recommend for ferrets.