Discuss Fitness: discussion board for fitness enthusiasts
International Health, Racquet and Sports Club Assocation (IHRSA)
Represents health and fitness facilities, gyms, spas, sports clubs and suppliers worldwide.
Sporting Goods Manufacturers Association
State Health Facts
Tennis Industry Association
You'll need to pay for most of the reports on this site, unless you're a member, but here's the free stuff.
American Journal of Health Studies
Since this site doesn't offer search, use the name of this publication when searching your topic of interest with a search engine.