Another option, not single leaf, were stiffer ones for rally use that were slipper springs where one end slid in a holder.
e.g. http://www.burtonpower.com/5-leaf-slipp ... ls288.html
Not a recommendation, I don't know anyone personally that has used them.
I just knew some of the rally cars I used to watch that did not go to the full 5 link with coilovers were using them.