A common source of names is records that have been compiled in some fashion. Compilers may acquire their names through public and private records such as vehicle registrations or phone books. Response lists are from people who have inquired or ordered through the mail. They are more mail responsive to your direct marketing offer. Another common source of names is magazine subscribers .
Trade associations are usually an excellent source of mailing lists.
Better associations always contain the industry's major players. Local associations like the Chamber of Commerce are good for local business names. Trade Show lists are also a good marketing tool - lists of both attendees and of exhibitors. Two excellent resources for investigating lists at the library are the SRDS Direct Marketing List Source and the Oxbridge Communication, National Directory of Mailing Lists. They're thorough and exceptionally easy to use.