Here is a selected list of Library of Congress subject headings which may be useful in your search. Check also for other useful subheadings, such as Bibliographies, History, Handbooks, etc. Guidebooks to cities may also have sections on city planning history.
If UC Library Search isn't enough -- particularly when your topic is geographically remote from California --use WorldCat.org, a database of more than 540 million bibliographic records in 483 languages. Request items through The Library's Interlibrary Borrowing Service.
In addition to the indexes listed here, some newspapers are available full text on the Web; information regarding archives of older issues is sometimes available at these sites. See the Library's home page→How to Find→Databases→ Databases: By Type→ News Articles Databases for a list of links to online full-text local and international newspapers and other news media.
Use journal and newspaper indexes and databases to discover articles on your topic. Some online databases include the full text of the articles you discover or the "Get it at UC" link which may lead you to the full text elsewhere.
Here's a selection of historical and current journals containing articles relevant to city planning or urban history.
Garden cities & town planning.Official journal of the Garden Cities and Town Planning Association .
Journal of planning education and research. Association of Collegiate Schools of Planning.
Planning history studies. Society for American City and Regional Planning History. Richmond, VA: The Society. 1996-1998.
Storia della citta. Milano: Electa Editrice. v. 1(1976)-56(1990) Nicely illustrated with plans, maps, etc.