I found this idea on Pinterest.  I tried it.  It works.

Take your empty Foaming Soap Bottle.
Add a small amount of
liquid soap refill.
This is sold in grocery stores next to bar & liquid soaps.
Fill the rest of the bottle with water, leaving some air at the top.
Tilt the bottle right then left to blend (shaking will cuse it to
bubble).

Done.
"Everything should be made as
simple as possible, but not simpler.”
Make your own refill for foaming soap.
bottle of Liquid Soap Refill
..................Fill soap to here
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..................Bottom of bottle
..................Fill water to here