I had purchased the front and rear suspension which included the leaf springs from Kris Kincaid/Roaring Sixties Motorsports
www.roaring60.com/roaring60/ try there... but I have done some homework too...
Here is a few spring manufacturers to try... this won't be a stock item you will likely need to review #leaves/dimensions/rate... (for that try a leafspring search of this site, it's here somewhere)
www.eatonsprings.com/
alcanspring.com/
These folks do list leaf springs in their catalogs.
www.motorsportsintl.com/
cobrarestorers.com/
Also Accurate Machine Products... can't find the link or as mentioned Kirkham.
Good luck...
OBTW aren't those plans great.
-John