To find clinical trials, search National Institute of Health's Clinical Trials registry and York Health Economics Consortium's site Finding Clinical Trials.
Examine reference lists: when you find relevant studies, look at the works cited to find more studies to include in your review.
A citation search can also help find additional studies. Eloise Flood at Pace University has a great guide to help with citation searching.
Perform an Internet search: using a variety of web browsers, search for your topic. This may lead you to individuals or organizations who have done studies pertaining to your question. You can then contact these people and places to see if they have any unpublished studies you would want to include.