If you've got a place that sells a lot of trailer parts and such, check there, see if they've got anything that can help...

I found a steel replacement leaf spring for my old Lumina Z34 (comes factory with a fibreglass mono-leaf) at Bradvin Trailer Sales... a heavy duty (think semi-trailer) parts shop...


And Trailer Replacement probably means it's a little stiffer, so the truck doesn't sink as much when you bolt a trailer on...