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Abhidhamma Iddhipada Vibhanga
Visuddhimagga (search under 'roads to power')
Malaysian scholar Piya Tan usually has references to Commentaries: Iddhi,pāda Vibhanga Sutta
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I haven't found a traditional Pali commentary on this yet but here is a modern commentary by Piya Tan: Iddhipada Vibhanga Sutta (SN 51.20).
Piya Tan usually refers to multiple sources including the traditional commentaries, to write his own.