We have a few library databases that contain theses and dissertations, although your options here are fairly limited.
Dissertations for doctoral-level programs are much more likely to be found in library databases than masters-level theses, so keep that in mind as you search.
Your next option is to search Google, specifically targeting educational institutional repositories, or IRs. Many universities have IRs where faculty, graduate students, and undergraduate students can submit capstone work like theses and dissertations. It may be easiest to find theses in particular through IRs by searching Google.
In Google, add masters thesis (or even just thesis) and site:.edu to whatever subject you're looking for.
Adding site:.edu to your search limits the sites that Google will comb through to only sites that end in .edu - so, the sites of educational institutions. You will still have to try a few of the results to see if they really are theses and not dissertations (but you may even be able to tell from the search results list by looking at the gray blurbs under each result).
Citation managers are sometimes good tools for these types of notes, but using a text document or spreadsheet and folders on your computer, OneDrive, or Google Drive can work just as well. The important thing is finding a system that works for you, and then using it consistently.
Citation managers can help you keep track of large amounts of sources, as well as generate references lists and in-text citations. They can save you a lot of time and effort as you compile resources and integrate them into your writing.
Zotero is a free bibliographic manager available as a software plugin in your web browser. Zotero collects, manages, and cites research from multiple types of sources. Through Zotero you can organize your research into different folders for different projects. Zotero can create bibliographies using Word or OpenOffice. Zotero provides a series of tutorial videos to get you started.
Mendeley is a free reference manager that allows you to manage citations and PDFs. It includes plugins for Word and OpenOffice to create citations and/or bibliographies as you write. Unlike other similar tools, Mendeley is a social networking tool that allows you to connect and share with other researchers and students - you can even login via Facebook.
This greatly expands the number of books available to you for your research. In the library catalog: