The most popular fill type for custom throw pillows is a Poly Bun. This fill type is Direct Fill blown into a cotton inner pillow (bun), which is sewn shut. Polyfill is mold and mildew resistant.
The Poly Bun is placed inside the pillow shell, which is made in the fabric of your choice. A hidden zipper on the fabric shell allows the inner bun to be removed for cleaning or in order to switch pillow covers.