The standard place for all (La)TeX relevant packages, classes and related code is the Comprehensive TeX Archive Network (CTAN). It contains all available source code as long the authors of it uploaded it to it. The main distributions TeXLive and MikTeX use it as their source of packages. There are some classes suitable for Ph.D. thesis, etc. However, there is not specific "template" category as far as I know.
The issue with templates is that there are so many different possible styles and so you a template is either very general and very specific, which in both cases doesn't make it very useful for a wider range. For example a Ph.D. thesis will look quite different in different countries or even for different colleges in one country. Not to mention between different fields of study.
I put a template for German final year project reports ("Diplomarbeit") which a made for my own Diplomarbeit some years ago:
(I actually have to review it again, my TeX-fu is now much stronger)