When I was building my Marlin Roadster with Triumph front suspension I spoke with three spring suppliers who were very happy to create bespoke springs for me, A range of wire sizes, diameters and lengths. The cost was, I thought pretty reasonable too, from memory form £80 to about £115 for a pair, with a very quick turn around, about 3 days.
http://springcoil.co.uk
http://www.coilsprings.co.uk
I can't find the third link but if you search Google for "car coil springs", you may find others.
As it happened I didn't use either company but they seemed very helpful.