There are a few ways in which you can obtain that information. The easiest one would be for you to contact the client you would want to have. Find out from their Marketing Manager or Director whether they have an agency at all. If yes, request to be on their list of agencies they would notify when the next pitch comes up. If not, request to pitch them. Most big companies and corporates have agencies with contracts running anything from 3 - 8 years. This means you'd have to wait till these contracts expire and the companies put out tenders for the next pitch. And another way would be the good old networking in the right circles and keeping your ears on the ground as it can happen that a company "fires" their agency prior to the expiry date of the contract and would then need an agency urgently.

i started operating a design & branding agency full-time as of January. at that time i also though that i need this information. turns out i don't. NEVER WILL.

ALL YOU DO IS DAMN GOOD WORK FOR ONE CLIENT. CEOs AND DIRECTORS PLAY GOLF AND GO TO TEAZERS TOGETHER. THEY TALK. YOUR NAME IS BOUND TO POP UP SOMEWHERE.