Way back in 2008, (16 years already?), I posted this.
At that time, to get your hands on the Zhuang lemons you had to make them yourself or befriend a passing Zhuang, my preferred method.
Since then, spurred on by the success of the restaurant chain I visited for lemon duck, some of the commercial sauce makers have started offering S: 咸柠檬; T: 鹹檸檬 (xián níng méng), salted lemons alongside their other sauces and condiments etc.
Also, I have discovered that the people of Chaoshan, mentioned above and in the Fish Sauce topic, also use salted lemons.
They even suggest adding slices to 7-Up and Lilt. I'm sure the Zhuang don't go there.