Couple of suggestions and they are both from past experience plus a bit of education.
Most springs are of similar metallurgy. Strength and rate come from three dimensions. Diameter of wire, Diameter of coil and either overall length or number of coils.
With that knowledge here is the two routes I usually take.
1. Auto parts store/ good hardware store etc. Some of these places have a spring assortment all sorted out and organized in one place. I think even the bigbox stores like Menards have these.
2. A dealership that has a parts man that knows his stock and is willing to look for you.
Nother option I thought of as I was writing this, do you have a AGCO dealer that handled Oliver back in the day. They might be able to help.
Should be doable though. It is a straight spring and not huge so my first try would be the hardware assortment I listed in option 1.