When you have a need, you'll know who to call, you'll ask informed questions, and your request will get priority treatment. For this to happen, lead time before a specific need is vital. If you haven't contacted a firm in advance, get in touch with it when you need specific information for a client engagement. Explain who you are, what you do, and what you need. Even if they never heard of you until that moment, you should get prompt and courteous results.
Most corporate switchboards have lists of departments as well as lists of people.
If the switchboard hasn't a clue, ask them for the PR group. PR and CR often work together, so PR will know where to route you. Alternatively, if you have had contact with the local sales force, ask them. Most consultant relations groups make it a point to keep their sales force aware of the consultant's role in the selling process, and of their own support programs for consultants. Salespeople are delighted to get you out of their hair and into the hands of professionals.