All of plays which you may use for theatre productions or study can be found in the country the author is from; so: for Beckett who is from Ireland you can look in Irish Literature which is roughly call # PR6003
For Shakespeare's plays, in English literature, the call numbers are for each play but roughly you can find them at PR2000 etc.
The easiest thing is to look for the title of a play in the catalog and just browse around from there.