The one in the Henne Mines is repeatable, so you could theoretically obtain infinite Zodiac Spears.....but why bother? It wouldn't hurt to say that the odds of obtaining it can be reduced to 1/100 via the following method: Zone in and out of the area until the chest actually appears, save your game, and open the chest. If you received the Spear....epic, if not, reset and check again. The chest will always be there, which reduces the odds down to 1/100.