There are several steps in the selection of subjects for a science project. Start with age. Do you want to include people of all ages? Or only certain ages such as 21 to 50 or over 50. What about gender? Should you include males and females? Or do you want a group of only one gender (males only) to control variables such as hormonal changes?
Will you include people who have been treated for their tennis elbow? What if they are currently receiving treatment of some kind? Will that conflict with the data you're trying to gather?
You'll need to select a sample size. Your science or math advisor should be able to help you use the formula (or software) that calculates what number would be statistically significant. Subject size may be affected by what you are going to use as a measure of success. Will success be defined by the amount of pain relief obtained? Days missed at school or work? Patient's level of quality of life?
Once you have all the variables defined, then you can start advertising. You may want to put flyers or posters up at health clubs or fitness centers, factories, and golf or tennis clubs. Depending on the ages chosen, you may want to put notices in college newspapers. Other places to post your need for subjects may include the public library, local hospitals, and even the public service announcements of the local newspaper.
It's best to get more than the actual number needed. There are always people who don't finish the study for a variety of reasons. Good luck with your efforts!