Dear Jyoti,
Generally, there are two ways to approach this problem:
1. Construct a new scale for the construct: Since you have not found a validated survey-based scale for Sustainable Financial Inclusion (SFI), you can develop one using an inductive scale development approach. This process typically involves item generation through literature review or expert interviews, followed by item purification and content validity assessment. You can then use Exploratory Factor Analysis (EFA) to establish the factor structure and Confirmatory Factor Analysis (CFA) to validate it.
2. Use a proxy variable to measure it: You mentioned that most existing studies use secondary data and indices for SFI. Methodologically, it is acceptable to adapt an existing Financial Inclusion measurement scale by modifying items to incorporate the sustainability dimension. However, if you choose to adapt a scale, it is crucial to ensure that the modified version captures the specific nuances of "sustainability" relevant to your study. You should also conduct a pilot study to test the validity and reliability of the adapted instrument in your specific context.
Best wishes
Neeraj