There are several sources and books that can provide sample case studies and model answers for practice in Agriculture Optional. These resources can be helpful in understanding the concepts, improving problem-solving skills, and familiarizing oneself with the type of questions asked in the Agriculture Optional paper of the UPSC examination. Here are some recommended sources and books:

1. "Agriculture at a Glance" by R.K. Sharma and S.K. Bhoi:
- This book provides comprehensive coverage of various topics related to agriculture.
- It includes multiple-choice questions (MCQs) and descriptive questions for practice.
- The book also offers model answers and explanations for better understanding.

2. "Objective Agriculture" by S.R. Kantwa and M.P. Pandey:
- This book is specifically designed for competitive examinations like UPSC.
- It contains objective-type questions with detailed explanations and model answers.
- The book covers a wide range of topics, including agronomy, horticulture, soil science, and animal husbandry.

3. "Agriculture Optional: Solved Papers" by K.N. Pandey:
- This book is a compilation of previous years' question papers of the Agriculture Optional paper in the UPSC examination.
- It provides detailed solutions and model answers to help candidates understand the pattern and requirements of the exam.

4. Online platforms like EduRev:
- EduRev is a comprehensive online learning platform that offers a wide range of study materials, including sample case studies and model answers for Agriculture Optional.
- It provides access to a vast collection of practice questions, mock tests, and previous years' question papers.
- The platform also allows users to interact with experts and fellow aspirants to enhance their learning experience.

5. Agricultural Journals and Research Papers:
- Reading research papers and journals related to agriculture can be immensely helpful in understanding real-life case studies and their solutions.
- Journals like the Journal of Agricultural Science and Crop Science can provide valuable insights into current agricultural practices and challenges.

By utilizing these sources and books, aspirants can gain a better understanding of the subject, improve their problem-solving skills, and develop effective strategies for answering questions in the Agriculture Optional paper of the UPSC examination. Remember to practice regularly and analyze your performance to identify areas for improvement.