Resource Review: the Choral Public Domain Library
The first resource that changed my life as a choir director was the Choral Public Domain Library, a huge crowd-sourced repository of public domain choral works. As of this writing, they host scores for over 40,000 works by more than 4000 composers, all free for downloading. Nothing on the website costs money, and you do not need to sign up to acquire anything.
With the multi-category search tool, you can filter your search results by language, voicing, musical era, genre, accompaniment, and more. Most composers have biographies and historical context pages, and translations are available for most common texts.
After using the site for a while, you'll start to find editors that you trust - since anyone can upload their scores, you're bound to find errors in your music sometimes. Some editors clearly have priorities other than ease of reading the score, so check multiple editions for staff spacing, readable fonts, and so on. My favourites are David Fraser, Brian Marble, Rafael Ornes and Abel Di Marco, but you'll find your own, depending on what you're looking for. You'll even develop your own preferences for what constitutes a "readable score." Does your choir have fits when the unit beat is a half note instead of a quarter? Avoid David Fraser. Do you like to have a translation and complete edition notes at the end? Try Rafael Ornes.
You won't find many hymns on here; I use this site to find meditation pieces for the choir to sing after Communion. There are plenty of great little gems in English on here, if you have a choir or congregation that is resistant to Latin - try looking through the works of William Byrd or Thomas Tallis, for a start.