I'm not sure I've ever seen an actual set for the 16" rims. The standard Farmall Cub weights were sold to go on the narrow, ag-style rims for the number series lo boy tractors.
From a technical standpoint, you really woundn't want to add weight to a rim/tire setup to provide displacement of weight, or low ground pressure. The alternative to this would be if you want to use that tractor and tire setup for a fron end loader. In that application, you would be much better off adding liquid ballast to the big tires, as you'll never add enough steel weight to the rear of the tractor to make up for what you could get by adding liquid weight to the tires.
If I were using a Lo-boy for plowing snow, or field plowing, and wanted more traction, I'd invest in a set of standard ag wheels and tires and a set of the standard F-cub weights. Then you could put the weighted tires on when doing "ground engaging" work, and swap them out for the high floatation tires for mowing purposes. Sounds like a lot of work, but it would give you the best of both worlds.