These would be the common methods:
1. Contact them directly - but make sure you offer something in your email that makes it worth their while to answer your questions.
2. Run a paid/free survey (lots of platforms offer this service - just focus on your target market). If it's a free survey, offer a prize (and give it) to one of the participants.
3. Try using Google Alerts for variations of the phrase "company retreat"
Obviously you would need to know what you are doing with each of the above options so that your results are accurate. The wording in each question is important.
I've successfully helped over 300 entrepreneurs and I'd be happy to help you as well.
I am assuming that you are in the corporate retreat management business?
If yes, I would start within your own geography since you'll probably have to do face-to-face selling and onsite delivery. I would target companies with 500+ employees. Within those companies, try to get to HR and any internal event planners. Companies with a large, active sales force is also a good target. Even is business is down, sales meetings are the last thing to go.
Set-up a call and I can get into more specifics once I learn more about what your company offers.