You mentioned Summit who are my go to guys for the vintage tires these days. They offer free shipping to the border, which works out well. At least it will when the border opens. Their price is $367 Canadian shipped.
https://www.summitracing.com/int/parts/cok-77240
The farm implement tires, if cheaper, might well be the way to go since you are unlikely to be towing this configuration down the highway. I can't speak to the relative lifespan(weathercracking) of an implement tire vs a highway tire though.