Financial resources for M.Sc. students in the U.S. universities
This depends a lot on the university, the discipline, and the individual department.  

A key factor is that many universities have "terminal masters" programs that are not research oriented.  Students in these programs typically complete an MS degree by coursework only, and don't complete a thesis.  Students in such programs almost never receive financial aid in the form of fellowships or teaching or research assistantships.  Other MS programs are research oriented and include a thesis or significant master's project.  Some departments have two different MS programs, one with thesis and one without.  

Even if the MS program is research oriented, departments with strong PhD programs tend to focus on supporting PhD students and MS students may either not be eligible at all for assistantships or they may be given lower priority than PhD students.  

Some departments that do not have PhD programs do have research oriented MS programs and will provide financial support to MS students.  This is one option, although these departments are not going to be as highly regarded as the departments with strong PHD programs.