Reference: CMP seat back coil spring dimensions

[Grant Bowker]

The "by the yard" springs we found that were closest to the springs we were finding in seat frames for OD and wire size (and thus stiffness) were listed at McMaster-Carr, their product 9664K52 https://www.mcmaster.com/#catalog/123/1277/=193uptc
Oddly enough the springs were labeled as made in Australia so Australians may be able to more easily track down the source and not worry about McMaster's policy of not selling to those requesting international shipping or likely to take the parts out of the US. (so much for their argument about protecting US technology from intellectual theft).
To re-bend the ends for attachment to the frames and rings/washers, hand tools were adequate. The original(?) springs we found in the frames had similar (the short style in the photo posted by Hanno) hooks on each end (differences due to age and abuse?).