If you know of an article on your topic, you may use Citation Linker to locate the article.
Always look at the References list at the end of each relevant article you find, to see if any of the listed sources might be useful!
Google Scholar is useful when searching citations. The Advanced Scholar Search option is the best way to search for articles which cite a known article. Check the "Cited by #" link below a search result to find additional useful sources - more recent articles which cite that relevant or classic article.
More ways to search citations may also be found in the Citation Searching section of Searching the Scientific Literature.