Since a lot of members here use Summit, here is info straight from their latest catalog.

Afco oval track springs, mono leafs, $55 ea, 5" arch. Available in 34, 47, and 64 spring rates

Multi-leaf Chrysler style spring packs $130 ea
5" arch available in 142, 152, 166, 194 # ratings.

Another vendor worth checking out is Landrum. They also specialize in leaf springs.

So it isn't that tough to get specialized spring rates at a reasonable cost from a vendor who knows what they are selling you.