Try Bill's Truck Shop in Ontario, Canada. I linked the catalog for 67-72 parts below, I think the springs shown on page 47 (with price on page 48) are what you are looking for. When I rebuilt my bench seat I bought a length of spring, similar to what Getter Done posted above, from an upholstery shop and bent it into the correct shape. This was before I found out that Bill's sold these springs.
https://billstruckshop.com/wp-conten...2-2023-Jan.pdf