Any competent spring shop can fabricate new leaf springs to match the stock springs.
IH came with a couple "variations" -- std and heavy duty plus there was hd with auxiliary springs (rear). IH "rated" the springs on "load capacity" -- 2800 lbs, 3500lbs, 5000lbs, etc. If you have the line setting ticket for your truck, the "load capacity" of the original springs will be listed.