USAToday provides data on median team salaries for MLB, NFL, NBA, NHL, and MLS.  Data is available from 1988 through the 2011 season.  It also provides salaries for the top 25 players on each team.  You'll also find teams' schedules, rosters, transactions and other data on the site. Choose a sport, then click on 'statistics' for the data.

SBRnet - Sports Business Research Network
A continuously updated resource providing market research and industry news on all aspects of sporting goods, sports equipment, participation, broadcasting, and marketing. The information is drawn from journals, newsletters, government reports, and trade association reports.

SPORTDiscus
Provides citations and some abstracts from periodicals, books, conference proceedings, dissertations and theses, research papers, etc. SPORTDiscus covers the areas of: sport, physical fitness, sport science and recreation.

LexisNexis Academic
LexisNexis provides company history, profiles, financial data, SEC reports, and company news. LexisNexis includes full text articles from hundreds of newspapers, magazines, and trade journals.

Business Source Premier
Provides access to articles in numerous business and news publications including the journal Sports Marketing.

Market Share Reporter Ref. HF 5415.2 M37
An annual compilation of reported market share data on companies, products, and services. Also try Business Rankings Annual, Ref HG4050 B88.

Sports Business Journal
Located in Current Periodicals at call number GV561.S78. It covers the business side of sports.