AIP ENR 1.5-33 details QNH sources as
a. Actual aerodrome QNH
b. Forecast Terminal QNH
c. Forcast Area QNH
As for distances I can find no reference in AIP for remote altimeter setting procedures. I do note that PANS-OPS allows the altimeter setting station to be up to 5nm away from the landing threshold before corrections need to be applied during the procedure design process.
Practically, I would say that if the altimeter setting source isn't at the aerodrome, then the altimeter setting is not valid to reduce MDA by 100ft.
Cheers,
Alpha