Take some bits from yours along, match up the wire thickness.
Measure how long yours was if all the bits are there, or measure the space you need to fill plus a little bit.
Buy something close, and a couple washers or bushings with the wide rim, sometimes called fender washers, that fit over the shaft and bigger than whatever spring size you buy.
Put a washer, the spring, a d a washer on the shaft.
Enjoy using the plow.
Most springs with the same size wire and same length will be about the same strength. For this use, it probably isn't all that critical, would be different for internal engine parts or so, but here - close is pretty good.
Now if you want a show piece with exact replica then I understand, you will want to source a barrel spring of the same dimensions exactly, I get that. But to use and enjoy, close is gona be plenty good enough.