Least evil way to make soap?
By far the largest source of lye nowadays is the electrolysis of concentrated solutions of $\ce{NaCl}$ or $\ce{KCl}$ in water, the famous and incredibly industrially important chloralkali process, producing millions of tons of $\ce{NaOH}$ and $\ce{KOH}$ every year. While electrolysis is an energy-intensive process, presumably the electricity could be supplied by a low-carbon energy source such as solar or wind power. 

"Replacing soap", in the sense of halting the usage of surfactants, is not a good idea and likely not even possible. Surfactants are too useful a class of substances. In fact, soaps are likely the most environmentally friendly surfactants, given that they're based on a simple hydrolysis and acid-base reaction with common biomolecules, fatty acids and triglycerides. Other surfactants such as alkylbenzenesulphonates (dishwashing liquid), tetraalkylammonium salts (hair conditioner) and perfluoroalcoxylates, among others, can be toxic to aquatic lifeforms and take longer to degrade in the environment. The excessive usage and improper disposal of surfactants is a recognized environmental problem, but tackling soaps won't really get you far in solving the issue.