Soap isn’t what I would consider a necessity. If you survive the looters taking all the soap from dark grocery stores you can probably find someone who knows how to make soap. Until then you can bathe in water, assuming you can find clean water to spare, because drinking water is more valuable than bathing water.

Pretty much everything you need to get by in the imaginary post-apocalypse world (which will be littered with consumer goods for a long time) you can find the Boy Scouts Handbook. The normal human solution to not knowing how to do or make something (historically) is to find someone who does and partner or barter with them. Trying to collect and carry around all of the world’s knowledge seems the least plausible way to go.