I use the Full Voice Workbooks, which can be purchased through Amazon for our theory and ear training. These books help to train many different aspects of each singer and equips them to be ready for many new experiences they may encounter as a singer. https://www.amazon.com/s?k=full+voice+workbook+series&crid=2Q345Q8J5PKN4&sprefix=Full+voice%2Caps%2C165&ref=nb_sb_ss_i_1_10
I also use the Royal Conservatory of Music Voice Series as our standard repertoire book. I find these books provide a wonderful diverse collection of pieces which can help establish and work on good technique. https://www.rcmusic.com/voice
I also encourage each student to bring in one or two pieces that they simply love to sing. I always tailor these pieces to each student. A few good resources to purchase individual songs are:
https://www.sheetmusicplus.com/
Workbooks and Repertoire books
Pencil
Water
Separate binder with any printed music and blank paper for notetaking
Device to access any downloaded accompaniments that are not able to be accessed by me.