When you're in for a long session of meditation, comfort is going to be important. Back strain, leg strain, and other physical injuries can result from long periods of even moderate strain on joints, ligaments, and muscles. Circulation is important, especially in more mature individuals, to avoid the possibility of a stroke or blood clot. Thus, many people utilize the zafu, or a sewn seat, as a way to maintain comfort and posture while meditating.
Specification
zafu cushions
It is usually a round pillow about 16 inches in diameter and about 8 inches high, though designs differ widely. The outer skins comprise a circular top and bottom, which are connected by a pleat, says Anthony Flanagan, a writer and practitioner who provides guides for people on Zen philosophy. He says that this material is usually made from cotton drill or cotton twill and can come in all sorts of colors depending on the supplier. The filling can vary too, but the most common is kapok, a natural fiber taken from the Ceiba tree, which is grown in the far east, or buckwheat hulls. This material is used because it enables the cushion to keep its shape despite frequent sittings. Most of them come with a concealed opening (so that the filling can be taken out while the outer skin is washed) and a carrying handle.
Position
zafu for meditation
The cushion is placed under the buttocks, while the user is sitting cross-legged. This allows for a physics dynamic that pushes the knees toward the floor while keeping the circulation going in the legs. It also cushions the body's weight on the legs. This seat is often used in conjunctions with zabutons, a long cushion that is used to further comfort the legs.
Though the fundamentals of meditation do not often include some degree of physical comfort (as people can sit on beds, couches, and other modern conveniences), this seating arrangement can still be a useful tool.

Flanagan says, Zafus and zabutons are found more usually in the Zen tradition, but they have gained wider currency not just in Buddhist meditation but with health clubs and fitness centers. It is important to note that they are not essential to good meditation. However, they can contribute to its efficacy.
Cost and Availability
zafu for meditation
Commercial enterprises offering zafus for sale are on the rise. For those seeking a combination piece with the zabuton, there are a host of manufacturers offering different color combinations and styles. Samadhi Cushions offers individual ones from about $40 to $90, though they do also offer some kits that include pairings of both the seat types for about $108. Sage Meditation has similar offerings available, though their prices are slightly higher.
So, if you are addicted to meditation and wish to do it in complete serenity and comfort, go ahead and buy a zafu right now. They can be used for a variety of other purposes beside only meditation.