I would start with the Yellow Pages. If you can't find what you are looking for there, I would contact the larger advertising and marketing companies since they will know. You can also often find company's names on the lower frame of the billboards. Then you can look up the name in the YP.

Nielsen Technical Services
http://www.NielsenTech.com
http://www.Consultant-Directory.com
http://www.EbayAuctionAds.com