But most are just printed up as technical reports by their respective CS departments and can be ordered that way. But note that not every department prints dissertations as technical reports. Some departments also publish dissertations through University Microfilms. If they have the dissertation you are interested in, they will photocopy it and mail it to you, for a fee. You might want to ask your reference librarian first about how to order stuff from University Microfilms, since I've never done it.
Interlibrary loan is another option for obtaining theses.
If it's a thesis produced at UNC, then one of the libraries on campus must have a copy. All CS theses are maintained on the upper level of Brauer. If you can't find certain journals, books, or conference proceedings here at UNC, you might be able to find it at Duke or NC State. Even if you root with all your might against both schools at football or basketball games, you will probably have to visit one or both to hunt down missing references. Because both Duke and State have an engineering program, and we don't.