To search a thesis in Google first go to Google site, enter "university search" in the search box. Now a page(from the Google itself)will appear on which you can see the word UNIVERSITY SEARCH. click there. Then a page will appear with a lot of university name in it.Click a name.Again a search box will appear. Write what you want to search in it,for example,you can write "etd"(stands for electronic thesis document).You will be taken to that university page(actually on their online library page). You can find a lot of thesis on the renowned university e.g. Caltech, MIT etc. It is possible to download most of the thesis from there though you can find some thesis as protected.
I downloaded some thesis from Caltech and you know, a good thesis can contribute a lot, sometimes even better than journals.
Hope you enjoy a good thesis hunting.