Decided to freshen up my 1973 240z the front with new bearings, rotors and brakes. Pulled off both sides and it looks like one side is a 260z hub. I have been driving the car and haven't noticed any

out of the ordinary except a steering wheel vibration a 50-60 mph, which I thought might be bearings/balance. Could the different hubs explain the vibration? Where can I get a 240Z hub<

Try zcarsource.com

Couldn't he go the other way also and get another 260Z hub.  Might be easier to find.

That first picture implies that the hubs place the wheels on different bases.  But as I understand it all S30's have the same wheel base dimensions.  A better way to take that measurement is with the hubs on a bearing and spindle since it's where the bearing sits that will determine where the rotor ends up.

In short - is there really a functional problem?