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)
Shawn Thomas (USA)
QSVP (Canada)
Aaron Dale (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