In my opinion, there is NO substitution for Masa Harina. Masa Harina is a flour like powder used to make masa. It's made with sun or fire-dried corn kernels that have been cooked in limewater (water mixed with calcium oxide). After having been cooked, then soaked in the limewater overnight, the wet corn is ground into masa harina. I know there are different kinds of Masa Harina (there is a Carribbean form), but I don't know of any substitute. Pretty unique stuff.
Someone else here may have an idea for you, though.
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