Each university will have its own policies on the subject. In general,
there are very few opportunities of financial aid available
to foreign students, specially at the undergraduate level. About
300 institutions give financial aid (generally partial) to international
students, and you can find information about them at the Educational
Advising Center library, in books such as "The College
Board's International Student Handbook" and "Peterson's
Applying to Colleges and Universities in the United States".
Roberto Volfovicz completed his Master
in Mathematics at Iowa State University,
and received a prize for his teaching of the subject.
At the graduate level, US universities may offer financial assistance
in the form of research or teaching assistantships. The
funding provided may be a tuition waiver or a monthly
stipend for board and lodging, or both. These assistantships
do not cover the cost of international airfare. To be competitive
for assistantships you must have excellent scores in the GRE
or GMAT, excellent grades, strong letters of recommendation
and a statement of purpose which is both precise and interesting.
The TSE (spoken portion of the TOEFL) and the GRE Subject Tests
are also helpful.