We at 180DC highly recommend using the free resources on Consulting Challenge to prepare for consulting interviews.

Consulting Challenge was created by a team of two current McKinsey consultants in 2018, and contains a compilation of the best, free resources available online to help aspiring consultants prepare for interviews specifically for McKinsey and the Boston Consulting Group. However, its insights can still be applied when interviewing at other firms. The site contains strategies to score higher on standardized consulting tests, cracking business cases step-by-step, strategies to approach PEI questions, a listing of free casebooks prioritized by difficulty and stage of preparation, and more.

Best of all, its resources will forever be free!