I buy them from NAPA or Tractor Supply or Rural King etc. A typical ballast resistor ohms range for many old 12 volt tractors using 6 volt coils might be in the range of 1.25 to 2 ohms.
Indeed if the resistance is too low and youre switching too much current in the points (4 to 5 amps is max on many points equipped tractors) they will burn up prematurely and the coil can run a bit warm.
If you wanted a 50 50 voltage divider where you drop 6 volts across the ballast and leave 6 for a 6 volt coil, their resistance would be the same as the coil. Many typical (not all) old tractor 6 volt coils are in the 1.25 to under 2 ohms primary LV range as likewise, many ignition ballast resistors.
With Ign ON and points closed put a meter on the coils input (looking for 6 to 7 volts, if too high, points will burn up quicker and coil get hot) and you will see how may volts the ballast dropped. Or you could just measure the voltage drop across the ballast itself.