One of the most simple Helm runewords is the Lore runeword which only requires ORT and SOL runes (in that order) in a 2-socket Cap or Tiara.
The ORT and SOL runes have a chance to drop in Act 4 and above; but if you are in Hell difficulty, you can also farm the Crypt and the Mausoleum in the Burial Grounds (Act 1). My Sorceress earned 4 levels farming these two dungeons a few times. The Countess has a chance to drop ORT and SOL in Nightmare and Hell difficulties.
I was also able to farm a Tiara helm in the Crypt.
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