Grants from federal, state and local governments — as well as corporations and private foundations — are the primary source of research funding. In addition to providing financial support, grants increase the credibility of your work and can make it easier to connect with other clinicians and researchers with similar interests.
The National Institutes of Health hosts a vast directory, the Office of Extramural Research (OER), devoted to federal biomedical research funding. Read an introduction to the OER.
Here are a few OER pages to help you get started:
See also the grants, funding and fellowships page at the National Library of Medicine's Health Services Research Information Central.