You can't buy the solenoid separately the whole thing is a single piece of kit. You've already been to the part you need to replace because you did a voltage check on the plug. You'll need to disconnect all of the connecting rods from the keyhole, the external door handle and the internal door handle and then remove it.

You WILL cut your arms to shreds because the inner door skins are sharp and the solenoid is a pig to maneuver out.