I don't really know enough about Caster lamps to tell you. I would try Sylvania or Philips, their HID lamps seem to be better than most other HIDs made today. Now, if you're looking for a quality MV lamp, Westinghouse Lifeguards are the way to go. eBay has tons of them for sale. However, those might be hard to find for cheap.
From what I understand Caster MV lamps are pretty low quality. For something relatively recently made in MV, EYE would also be a good way to go, too.