A data chart with dimensions of car suspension coil springs currently, and previously, sold by the Scandinavian parts store Biltema.
Can be used to search for a suitable custom spring for your build. (Swedish)
A gaming seat setup I designed for Logitech G25 and a Saab-seat, inspired by a Lotus super 7 chassis. 3D-models (igs or stp) and pictures are available, see links to the left.