Bear in mind that recruiters are always trying to improve their processes so it is likely that they will find new ways to assess candidates each year. It is therefore unlikely that you will find comprehensive resources to practise legal case studies in the same type of ways that you might for management consultancy case studies (in the consulting industry, case studies are extremely well established and follow a broadly similar format during the interview).
Generally speaking legal case studies are not intended to test out technical legal skills – this is partly because they need to be fair to both law and non law students. Instead they are more likely to test the skills required to be an effective lawyer within the world of (private practice) work. So focus your efforts on your specific law firm research, ways to demonstrate your motivation, and the practice of skills they are seeking rather than on preparing for specific types of exercises.