While the library catalog is the best place to check for specific items, if you just want to browse the shelves, the following call number ranges may be helpful
M1503: operas (piano-vocal scores)
M1507-M1508: songs from musicals, individual arias from operas
M1619: anthologies of secular songs
M1620: secular song collections organized by composer
M2110: anthologies of sacred songs
M2112-M2113: sacred song collections organized by composer
ML54.6: vocal texts, including translations
ML410: composer biographies
ML420: singer biographies
MT820-MT823: singing instruction and vocal technique
Competitions and auditions sometimes require you to use original scores, not photocopies or library volumes. Here, without endorsement, are a few sheet music vendors.