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I think on expert pretty much everyone has perfect IVs, but only some of the more elite opponents have perfect EVs. Personally if you beat the game before I'd say go with insane if you are using sandbox and want a challenge. On that mode your opponents will have competitive style setups equal to...
Steal them from wild Magmar (5% chance to hold), find one in Cinder Volcano (think you need surf first), or buy one for BP in the postgame. Wild Magmar are found in the higher level areas of Cinder Volcano that you can't get to your first trip. The hidden raid den is your earliest option I...