If you told us exactly what you're meant to be writing your dissertation on, we might stand rather more chance of being able to help you!
It's not always easy. When I was writing my Mathematics dissertation (in Sheffield) I had to do a great deal of research to establish that the two books I required were only available from one specialist library in London. (They were the only copies). Further, one of them was written in 18th century German, using a notation which was unfamiliar to me but (after a considerable struggle) I still managed to incorporate part of its content into my thesis.
There are at least 4 mathematics graduates regularly posting on this site, together with people who've got degrees in most of the sciences. If you tell us what it is you're working on, we might be able to point you in the right direction.