How to make baking soda

I'm in a place where buying baking soda is proving to be quite difficult^*, so I was looking for substitutions, and it seems there really aren't any. So this got me thinking: Baking soda is made (or otherwise harvested from nature) in some way.

How can I do this?

I realize the answer is likely to be so involved or expensive that I won't want to undertake the project, but for the determined baker, how would it be done?

^{*I eventually found some.}

NOTE: The question is about making, not sourcing or substituting, baking soda.