Bach chorales are a popular way to warm up a section or ensemble. Bach Chorales Deconstructed presents a set of chorales in solo, duet, trio, and quartet formats for convenient use in a variety of settings. This resource is equally applicable in the lesson studio, the orchestral sectionional, and the quartet rehearsal.
In addition, I've transposed 384 of Bach's harmonizations into trombone range and compiled them into A Trombonist's Hymnal, comprised of two volumes. These quartets can be downloaded for free under a Creative Commons license.