Separate boiler and AC, to which does C wire connect for Nest thermostat
Run it from the A/C air handler

According to this tech support article from Nest, the Nest prefers Rc and C over Rh and C when all three are connected.

So, I'd run a new thermostat cable (18/8 is trivial to find, although even 18/4 or 18/5 will do for what you're doing) in place of the old one from the air handler -- this gives you spare wires so you can use one for a C wire from the Nest to your air handler.  Once you have the wire in place (blue is the standard color), you can connect it to the C terminal on the Nest and to the #6 terminal (as you have identified in the diagram) on your air handler's terminal block.

From there, Rh and W1 on the Nest go to the red and white wires from the boiler cable, while Rc, Y1, and G go to the red, yellow, and green wires in the new cable to the air handler.  At the air handler end, the red wire goes to R (#8 terminal), the green wire goes to G (#7 terminal), and the yellow wire goes to Y (#2 terminal).