If all else fails ( and I really don't think you will find that bulb holder ), you can go to a scrap yard and find some sort of holder that will fit the hole and wire it externally to pin 1 & 2 of the headlight connector harness. Glue it in, or fashion a spring fork type holder screwed to the plastic housing.

( pin 1 is 12v from the flasher, pin 2 is ground )