Finding reputable online repositories and digital libraries that offer sociology optional study materials can greatly enhance your preparation for the UPSC exam. Here are some steps you can follow to locate these resources:

1. Research Online Repositories:
- Start by conducting a thorough online search using search engines like Google.
- Use specific keywords such as "sociology optional study materials" or "sociology digital libraries" to narrow down your search.
- Look for reputable online repositories that specialize in providing study materials for sociology optional.

2. Check Educational Websites:
- Visit renowned educational websites like EduRev, which offers a wide range of study materials for various subjects, including sociology.
- Explore the sociology section or search for sociology optional study materials within the website.
- Ensure that the website you choose has a good reputation and positive reviews from users.

3. Utilize Online Forums and Communities:
- Join online forums or communities where UPSC aspirants discuss study materials and preparation strategies.
- Engage with fellow aspirants and ask for recommendations on reliable online repositories or digital libraries for sociology optional.
- Make sure to verify the suggestions given by checking the credibility of the recommended sources.

4. Consult Sociology Professors and Experts:
- Reach out to sociology professors or experts who have experience in teaching or guiding UPSC aspirants.
- Ask for their recommendations on reputable online repositories or digital libraries that provide sociology optional study materials.
- They may suggest specific websites or platforms that are trusted within the sociology academic community.

5. Check Online Libraries:
- Explore online libraries such as JSTOR, which provide access to a vast collection of academic articles and research papers.
- Search for sociology-related journals and publications that may contain relevant study materials for your optional subject.
- While some online libraries may require a subscription or paid access, they often offer high-quality resources.

6. Verify Credibility and Quality:
- It is crucial to assess the credibility and quality of the study materials offered by the online repositories or digital libraries.
- Look for materials that are authored by reputed sociologists or academicians in the field.
- Check for reviews and ratings given by other users to ensure that the materials are reliable and comprehensive.

Remember to always verify the authenticity and reliability of the sources before using any study materials for your UPSC preparation.