Well, it is easy to find papers online, but If I need to search for any PhD thesis that has gone online ? how can I find it and buy it if necessary ? Thank you in advance.
The same type of search engines that you use for finding papers, books, etc. can also be used for PhD dissertations. It is my experience that some (many?) universities will upload their students' dissertations to their websites for free downloads. You might also try finding an e-mail address for the student author and asking for a copy - most of them are more than happy to e-mail you an electronic copy, as they are glad to have people interested in their research.
In majority of cases Thesis are available online and are free as per university rules. Researchers always publish papers and finally make thesis. In this way it seems difficult not to get good papers or thesis. Finally one should target best groups of the world pursuing such type of research and contact them. This information can come from recent reviews books papers etc.