Second, if you haven’t found what you were looking for, go for any sources which focus specifically on the country where that composer was published; in this case I offer you Cecila (under administration of the helpful IAML people). You can search for the composer, or if you know that they have been published by a specific publisher, but you wonder whether a certain piece has been published, search for the publisher’s archive.
Third, if still no luck: try Access to Archives; sometimes a local authority, a public library or a small research institute might have acquired your composer’s archive.
Fourth, if the composer has published, contact the publisher direct and ask them; if you cannot find contact details through a web search, go to the Directory of Music Publisher Association [UK] members: http://www.mpaonline.org.uk/directory; if the publisher has ceased to exist you can search ‘associated’ publishers at http://www.mpaonline.org.uk/associated_companies
Fifth, consider that – even though a composer might have been popular in their time – some music (even, or especially) in the last 100-150 years have been lost.
Have you found any other tools for, or paths to success? If so, please share them with a comment below.