Let us not forget the ancient method of making caustic soda from soda ash and slaked lime. Mix them in some water to precipitate calcium carbonate (limestone) leaving sodium hydroxide (caustic soda) in solution. 106 grams of anhydrous or 124 grams of monohydrated sodium carbonate react with 74 grams of calcium hydroxide (slaked lime) or 72 grams of calcium oxide (burnt lime) to make 80 grams of sodium hydroxide.
It's pretty hard to imagine slaked or burnt lime not being available at garden supply stores, or sodium carbonate not being available at swimming pool suppliers.