A musician friend just told me about Amidon Community Music, which offers a wide selection of SATB music. Many look suitable for use in UU congregations, and the website has testimonials from UU congregations. Prices are reasonable: US$5.00 for five copies. Of interest to congregations with a limited music budget, they also offers free sheet music downloads.
You have to enter your name, address, and email to access the free downloads; I haven’t yet taken the time to do that. But I’ve seen one of their arrangements, “Love Call Me Home” by Peggy Seeger, and it looks pretty good. My musician friend recommends everything they produce.