I only know of two ways to get them. Either by playing continuously in dark dungeon or buying them in limited items shop with money.
In the earlier years of this game, powerstones are dropping like normal stones and players are so lucky bcause of how easy it is to get. Screenshots of their legendary and unique swords have reached tier 30 to tier 50+ making their swords unbelievably powerful with newer players, having no chances at all to win in the arena and with cheaters adding to the frustration.
Today, powerstone is much harder to get, with many previous patches limiting their acquisition. But still you can get some, depending on how much time and stamina you spend in dark dungeon. Goodluck.