How can a decent student find a doctoral advisor from another faculty?

I am a last year student at the Moscow State University (I study maths). At our university there are two somewhat similar mathematical faculties: Faculty of Mechanics and Mathematics (this is where I study at the moment) and the Faculty of Computational Mathematics and Cybernetics - CMC for short.

After almost five years of study, when the time came to think about what will my PhD research topic be and where to apply for PhD study, I started to investigate possible options and eventually found that all the chairs that are close to my interest are of the CMC faculty. So started contacting professors from CMC, attending their courses, detailing my interests and asking them whether they are interested in teaching me etc.

To my dissapointment, all three persons whom I've contacted so far said that my interests are close to theirs, that they may have been interesting in working with me but, unfortunately, they have many students of their own, they have many applications and it would be difficult for me to find a PhD student position at their chair.

I also want to mention that I consider myself a decent student - I have a top mark in absolutely every course I've attended during my 5 - year study. Still, no one seems to want to even bother taking a "foreign" student (or even recommending me to some fellow scientists who may be less busy with students). I don't know what is the reason for this and how to overcome this, and after several refuses I feel demoralised and close to abandoning the very idea of a PhD study. Maybe you can give me some advice on what could help?

Thank you very much in advance.