Dutch theses can be found in UvA-DARE (UvA) and Netherlands Research Portal (all Dutch universities).
To find theses in print editions, search CataloguePlus or WorldCat Discovery.
The following reference works list older Dutch theses: Catalogus van academische geschriften in Nederland en Nederlandsch Indië verschenen (1924-1945), Catalogus van academische geschriften in Nederland verschenen (1952-1979), and Bibliografie van Nederlandse proefschriften (1980-1990).
Full-text theses may be downloaded from:
More databases with dissertations:
A number of subject-specific databases contain the data of dissertations. Each database has specific descriptors, publication types and options to limit the search results. In Sociological Abstracts, for example, you can limit to Publication Type Dissertations & Theses. If you find an interesting reference, the thesis may be full-text available through one of the resources mentioned above. If this is not the case, you might try ordering it from Dissertation Express.