The National Association of Teachers of Singing is a professional organization for teachers of singing. It's mission is "to encourage the highest standards of the vocal art and of ethical principles in the teaching of singing; and to promote vocal education and research at all levels, both for the enrichment of the general public and for the professional advancement of the talented."
At this website you can find information on student auditions, programs, events, and other items of interest.
Ruth Hennessy has developed a method of incorporating bodywork (such as Alexander Technique, yoga, and Pilates) to create the optimal vocal instrument- the whole body. At this website, you can explore some of the techniques and foundational principles of this method.
The Estill vocal method is based on Vocal Versatility and Vibrant Health. "There is no cultural nor aesthetic bias in Estill Voice Training, but there is a bias for vocal health! Estill Voice Training encourages those who seek vocal versatility to explore the full range of expression in the human voice, but not at the expense of vocal well-being. One way that Estill Voice Training does this is by promoting “Most Comfortable Vocal Effort” at all times."
This website offers information on basics of the method, events, certification requirements, and more.
This is one, of many, websites that sell sheet music. Hal Leonard, in particular, publishes many excellent editions of vocal anthologies.
This website offers texts and translations of thousands of vocal works.