Most farm supply outlets (tractor supply, etc) sell a paint called Massey Gray that is reasonably close. Unfortunately these are closer to modern shades than than the older shade but only purists would tell the difference. The Massey dealerships sell a paint for restoration work too, but its pretty pricey and I am not sure its really any closer than the over-the shelf brands.
To get historical accuracy, you'll have to go to a place like NAPA, PPG Industrial Coatings, etc and match an unworn patch of your paint. You can often find paint in good enough shape to match against on a piece that doesn't see wear, heat, sun, or water - like the bottom of a bracket that mates flat against housing somewhere under the tractor. I matched a red once this way and it worked perfectly. You might have to buff the paint with a rubbing compound and polish to make sure the automated spectral readers at these shops can get a good match.
Good luck,
Spencer