Vocal pedagogy resources are not only for people in the vocal pedagogy program. Any singer curious about theories and physiology of vocal function will find these books helpful. And if you're teaching lessons or leading a choir, you'll find tons of information in these books.
Sites like Google Books and the Open
Library (which aims to have a web
page for every book ever published) have many Public Domain available
online in full text.
For books not available in full text, a convenient link leads to sites where the book can be purchased. Check RuCat for library availablity or consider using Interlibrary Loan.