Many groups like to use learning tracks to assist in learning the music by ear. Similar to the list of arrangers, there are a vast number of people creating learning tracks, a small selection are listed below:

Tim Waurick(USA)
Peter Nugent(UK)
Antoine Kaiserman (UK)
Dale Kynaston and Andy Funnell (UK)
Tim Beutel(USA)
Tony DeRosa(USA)
Michaela Johnston(USA)

A lot of these learning track creators have a back-catalogue of tracks you can purchase quickly, and they also will accept commissions of new learning tracks.

Other Helpful Links

harmonize.com

barbershop.org/sheet-music-songbooks/

barbershoptags.com

barbershoptracks.com