Are there funding sources for open-access fees for an independent researcher in industry?
As a formerly independent researcher I have looked into this matter before; regrettably, the answer is disappointing: Funders usually require an affiliation with a research institution. (Yes, it is paradox that 'open science' generates new exclusions, leaving certain segments of science behind; this entrenchment of new inequalities due to open access-movements was recently called 'academic colonialism', though this term may go too far.)

Anyway, there may be an alternative route. So-called transformative agreements between scholarly publishers and (country-level) consortia allow researchers from all institutions covered by the consortia to publish Open Access (OA) for free.

Now if you are well-connected to researchers at research institutions which are eligible for this waiver of OA publishing charges because they are part of the consortia, then you may ask them whether you can "borrow" their institutional affiliation. That way you would be able to publish OA without any costs. This does succeed sometimes - because research institutions also have an interest in inflating their publication statistics.