Most companies that still make a 1/3 HP motor just stamp 1/3HP on a 1/2HP motor. However, it is the pump end that actually determines how much energy the motor draws. For instance Grundfos makes a 7S05-11 and a 7S05-8. Both are designated as ½ HP pumps. However the 8 stage used to be a model 7S03-8, which is only 1/3 HP.
So you can still get 1/3HP pumps if you know how to figure the load, you will just have to put a ½HP motor on it since that is all you can find anymore. And really, all motors have been shortened up over the years to promote planned obsolescence, so the 1/2HP motor is probably even shorter than the old 1/3HP motors of the past when things were made to last.