Local suspension repair shops are probably the best. Just about all they have is 5160 in used leaf srpings in the scrap bins, and yes it is a chore and dangerous to cut already tempered steel as when it gets hot it graps the cutting wheel and can rip it out of your hands, not to mention all the rust and pitting removal as well as looking for fractures in the metal. I think most of my common utility knife sales come from this stock, it is more work than new stock if you intened to forge & fold the scrap steel , you might also need a way to add carbon to the folding. If you have a plasma cutter all the better it works great on cutting leaf springs just remember to leave at least 1/4" spacing around your blank tracing.
Try to get the best pieces out of the scrap bin. The wider and slightly thinner transport leaf springs are generally of better quality steel. Thicker leaf springs are fine for folding but too much trouble for a straightened piece as a knife blade. I've picked up 4,6,and 8" wide leaf springs and thicknesses upto almost 1/2" .
When removing the rust with an angle grinder you first grind off the top rust orangish colour rust then rince with water. Then angle the grinder a little more to get the carbon like black pitting deposits. No need to get rid of all the pitting, standard cutting and belt sanding will take care of the small stuff. Wash off between grindings, let dry then coat the metal with 2 stroke engine mixing oil and let sit for a few hours.
Clean and start either flatening the steel or what ever process you want to try.
Safe Journey.