Games : Which is the best way to get plus eggs?

Plus eggs refer to Monsters who have (a) +(s) to their stats and are optimal to be Power-Up Fused to another Monster. Whether or not a Monster egg drops with a + is random, but certain events can increase the chance of them dropping. Which is a consistent & efficient way to farm plus eggs? Which is the best (combination of) event(s)? Do some dungeon s drop more than others?