Government resources and opportunities
You can find funding opportunities in a variety of government departments and agencies: almost every agency has them. Check them individually in organizations of interest to you.
Note that while many of these opportunities sound as if they are meant for students in STEM disciplines, they also may be open to students in other disciplines including business, communications, policy, law, etc.
Benefits.gov: lists some grants and fellowships, but is not comprehensive
Grants.gov: lists all current discretionary funding opportunities from 26 agencies of the United States government
Science.Gov: internship and fellowship opportunities in science
STEMgradstudents.science.gov: graduate student STEM opportunities
STEMundergrads.science.gov: undergraduate STEM opportunities
USAjobs.gov: can include some of these opportunities
Environmental Protection Agency (EPA) fellowships
National Academy of Sciences (NAS) fellowships
National Institute of Health (NIH) undergrad and grad, with a variety of institutions, US and UK
National Science Foundation Graduate Research Fellowship (NSF GRF): the biggest at NSF, but there are a variety of undergrad, graduate, and supplemental programs. Start here, and you can also search “scholarship”and “fellowship” for possibilities.
National Security Education Programs (NSEP): language and culture study opportunities
Presidential Management Fellows Program for advanced degree graduates committed to excellence in the leadership and management of public policies and programs
U.S. Nuclear Regulatory Commission (NRC): search “fellowships and scholarships”