A dissertation is different from an essay. In writing an essay you can take help from the text books while for completing a dissertation help may not be available in the classroom texts.
You need to scour through books, journal articles, attend conferences and collect papers. Help may also be taken from newspaper articles, government publications and online resources. Searching in an unsystematic way may not help you. The search should be very systematic and include information not available in the library catalogue.
Online libraries are of help when it comes to searching information for your dissertation thesis. Online databases rather than search engines will give you quality content that you are looking for. In most cases you would need to use a membership to access these databases.
Prior to investing money for access, check out the free resources that are available. This will give you an idea whether the database would indeed help you. You need to check out the database by searching the Resources by Subject. The online tutorial or Helpsheet that is available should give you an idea of the resources that are available.
After all your efforts if you still find the information missing you can take help of an inter-library loan. Inter-library loan is beyond the scope of this article but let me inform you that such a facility is offered by your library. The services are offered at approximately $2 and the material that you request will be available in a few days to a couple of weeks.
There is one very important factor that you should be aware of when writing your thesis. Plagiarism or infringing copyrights is something that you should be aware of. There are some general rules that you need to take note of in order to escape plagiarism. Try not to use words as they have been used in lectures, web pages, books and articles.
This amounts to copyright infringement and if your thesis is found to be copied by the plagiarism software, chances are you may have to edit a substantial section. Try not to use ideas of other people. This also amounts to plagiarism. You can however take ideas of other people and write it down in your own words not forgetting to mention it in the reference section or bibliography.
The ideas mentioned above should help you to come up with a dissertation matching the expectations of your professors.