How to convert Li-ion battery voltage to 1.5 V?
I had thought of a \$\color{red}{\boxed{\text{buck converter}}}\$ module, but in the market I cannot
find one that supports 4.2-3.3V in input and output of 1.5V.

They are fairly easy to find. For instance, this one springs to mind: -

If you need a little more than 500 mA at 1.5 volts then a different device is required. Here's a selection page from ADI that might be useful to you.
This one is good for 1 amp: -