Musicroom
Musicroom offer manly books and some instruments. They are one of the main U.K. suppliers for books and sheet music. There are always deals and discounts for teachers, choir leaders and students.
Sheet Music Plus
Sheet Music Plus has lots of digital sheet music available. They’re an American company so don’t order books from them if you’re outside the U.S. but their selection of digital sheet music is excellent.
Musicnotes
Musicnotes have a great interface which lets you transpose sheet music and their selection of digital music is very comprehensive. Their Pro subscription gives great discounts as does their teacher subscription.
Amazon
Amazon has lots of music books of course and there’s always some really good deals to be had from the independent suppliers. If it’s available with Prime then the next day delivery is always useful.
Thomann
Thomann not only have an almightily good selection of instruments and equipment. Their own brand products are vastly improved recently and their customer service is brilliant. You do have to spend over £99 to get free delivery as they’re in Germany.
Karaoke Version
Need a backing track making? Karaoke Version has a huge selection of songs which can be easily modified to make not just a karaoke track but a guitar, drum, piano, choir backing track.