Hey there, here are some ways we conduct research:
1) Through internal research teams: MBB have dedicated research teams, usually specialised in a particular field. They are very helpful in conducting in-depth research and gathering as many facts/figures as possible. However, they are limited in that they need very specific instructions and they don't fully understand the case context
2) Using internal company database: We keep a database of past cases (anonymised) and thought-leadership conducted at the firm. If you find a useful presentation that is relevant for your current case, you can also contact the authors listed to find more information.
3) Access to paid-databases & Reports (e.g. Gartner): Consultancies also have subscriptions to major research/report websites that often have very specific/detailed data.
4) Expert networks: These are an excellent way to gather niche or "subjective" information- e.g. "where do you see industry X going in 10 years?" There are dedicated expert-network firms that help consultants find experts that meet their requirements - cost to the project is usually $1000-1500 USD - and you thought PrepLounge experts were expensive!
5) Finally, we all at some point resort to some good-old Googling!
Hope that helps!