The absolute best place to see who specializes in what is the schools' web sites. Im not an expert in either string theory or cosmology, but I know (some) of the top places for string theory are Harvard, Princeton, MIT, Stanford, UChicago...the usual list. Other schools have programs in both of those areas (Berkeley, UCLA, UCSD, - I know USC has a string theory group). I think US News and World Report may have a ranking for high energy and astro program, although I'm pretty convinced this is ranking is (not only arbitrary but) irrevelent to the quality of experience and impact on your career the program will have for you. As I understand it, the singular most important factor is the individual faculty who will advise your dissertation- so look for someone influencial to your field.
One caveat- not to get your spirits down, but I know string theory is a NOTORIOUSLY DIFFICULT field into which to gain entrance (even more so as a foreign student), so it would be wise to have some reseach background in the field- not to say you shouldn't try.
Best of luck,