rule of thumb: since two resistor in parallel with the same value give half the resistance of a single one, if you put a 470K in parallel of the outer lugs of a 500K pot you'll have (more or less) 250K R equivalent, so, use a general 500K pot then solder across each single (hot to ground) a 470K resistor.
(beware that you will feel a different taper of the volume pot for the singles than the same without the parallel resistance)