Springs

Zig Zag or Serpentine Springs are commonly used on a wide range of furniture.
The are easy to replace, and relatively cheap.

Commonly used in Parker Knoll Furniture these plasic covered tension springs come in a variety of lengths with a hook on either end ready to be fitted to the Metal Fixing Plate on the frame of the chair.

9 gauge Coil Springs in a variety of sizes. Generally a 5" Coil Spring would be used in an average chair.