You can get information on service marks at Nolo.com's Law Center for Trademarks and Copyrights. Service marks are usually discussed along with trademarks.
Other places to get information on service marks and trademarks
Many law firms that practice intellectual property law have extensive information on their Web sites for the subjects covered in their practice, including information on patents, trademarks, copyrights, and service marks and trade secrets. To find these law firms, you can search for intellectual property attorneys at Lawyers.com or conduct a search on Google and Yahoo! for "intellectual property attorneys."