common problem, unfortunately, no spare parts that i know of and most have used an industrial grade superglue to stick the ball joint back on, thats if you're able to remove the ball end out from inside the head.
if you manage to get this far, file one side of the ball joint slightly flat which will allow easier swapping of the heads. don't file too much off or the head becomes really loose and wont stay posed and continue to drop.
Thank you for your response and tip about filing down the ball joint. That's unfortunate that this is a common problem. The ball joint is really stuck in there. I'm going to have to power drill a small screw and yank it out with nose pliers or something. Kind of odd that the opening for the full helmet head is a bit smaller than the image head. I'm using simple paper tape on the image head and shaped it to fit the neck slot. It's holding up for display purposes for now.