One of my trombone friends suggested I look at Brassworks4 for brass sheet music. They have SO much to offer! When you go to their website, you will see music for all brass instruments, chamber ensembles, as well as pieces from other publishers like Euphonium.com, Cimarron Press, and BVD Press. The catalog includes works for tuba-euphonium duets, trios, quartets, and then some!
Brassworks4 is led by Connie Schulz. She is quick to respond to emails and phone calls. Her goal for the company is to create new and exciting works for brass players of all ages. The literature is available to middle school, high school, and college players. The other nice thing Connie does is she sends some of her pieces for peer review through the ITEA Journal. I have published with her and I highly recommend contacting Connie if you have any questions!