There is no single source for a comprehensive dissertation search. WorldCat and Digital Dissertations include most American dissertations. Dissertations @ The Center for Research Libraries lends non-American dissertations to member borrowers. Library catalogs and specialized repositories contain other titles. Request any dissertation through Interlibrary Loan. Though not every title is available through ILL, it is worth a try to get it.
The Center for Research Libraries reviews all Grad Center ILL requests for loan or demand purchase of UK dissertations. If CRL finds the title accessible through EThOS or that it can be digitized free of charge (in approx 30 days), CRL will notify the requesting institution of its availability via the EThOS online venue. CRL will also place orders via EThOS and alert requestors when a dissertation is available for download. If EThOS requires a fee for digitization, CRL will place the order on behalf of the requesting institution and pay for digitization.
Use these to supplement searches in online databases. Historical information in print indexes is sometimes more complete (i.e. abstracts appear in print before 1980 in Dissertation Abstracts International, but are not currently online). Print indexes may contain earlier works not included in online databases.