If you are searching for plays your local library is not generally a good place to start. Even tertiary institute libraries tend to have older works on their shelves. Searching the web is good but for specific searches you can try:
They are attempting to amass a complete listing of every playwright and play, published or unpublished, in English since 1956. If you are a playwright (even unpublished) they would like to hear from you as well.
The Victorian Drama League has a large catalogue of playscripts conveniently searchable by type, length and the number of male and female roles:
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Comedy, corruption and sexy 1940s charm come to the Old Mill Theatre this June in the play Born Yesterday.