Are these rectangular or round?
If the former, then the tractor was fitted originally with Speaker 777's which were discontinued around 1973. They are highly sought after for restorations and because they came with glass lenses that withstand hotter bulbs.
Used sets and the occasional set of NOS units do show up on e-Bay now and then but they are not cheap.
If your tractor has round lamps, then the sealbeam units can be bought from any NAPA dealer or other automotive parts supplier. They are available in either spot of flood style.