Are single seat springs available? I would like to replace the broken ones in my front seat. Mine are cylindrical and all I've found so far are conical upholstery springs. Of course I'd need springs of different heights.

Try your local upholstery shop yet?

A straight wire compression spring is stone boat simple for a spring manufacturer. Google wire springs. There are a number of them that advertise one off custom or prototype springs made to order.........Bob

Try American Trim in Oklahoma (http://www.atrim.com) or Gilbreath (http://www.upholsterysupply.com/index.html)

Follow up: Thanks for ideas. Looking for "wire" springs is a better idea than "coil" springs; I didn't use the right words. Nevertheless, ready-made springs are all double-cone upholstery springs and mine are (were)cylindrical. Modern cars use mostly "no-sag" type springs. Upholstery spring suppliers seem unfamiliar with car seats. Custom manufacturers are willing to make a spring if I can tell them load (per spring), deflection, wire gauge, and spring rate as well as diameter and height. Since I don't know the answers to most of the questions, the engineers are unable to continue the conversation. I will continue my quest.

From your post I assumed you had some good ones for a sample. If so you you should be able to measure wire diameter and turns per inch (or total turns) along with overall dimensions. That should be enough info for a maker. If they need the rate just put one on a scale and note the pounds per inch as you press down on the spring. If you have a good one you could just send a sample for duplication.
If you don't have a sample, calculate load by the size of your butt, the amount of coils it will compress, and how far you want to sink down.........Bob

OK! Now I will tell 'em like it is! Thanks for info. Simple as it seems, your instructions provided answers to to questions I didn't know enough to ask and no one asked them of me.

You could pull springs out of an old mattress. They're usually not rusty, and often are the straight type, I think.