Sources of researchable problems can include

Researchable problems can arise from all these sources, so All of the above holds.

The researcher's own experiences often spark curiosity that becomes a problem.

Practical issues that require solutions push applied and field research forward.

Theory and past research reveal gaps, contradictions and unanswered questions.

No single source is the only origin, problems flow from several at once.

Reviewing literature systematically is the surest way to locate such gaps.