My local library uses overdrive for ebooks, which you can check out and either download for kindle, download as epub, or read online in your internet browser. I usually download to an eink reader, but if I’m reading on my phone I use the read in browser option.

Suggestion: if you plan to read on your phone, look in the settings to set the background and text color of whatever app you choose to something that doesn’t strain your eyes.

Estate sales are another option, though your mileage may vary.