Ah, Flickr, the photo sharing service has been drawing some significant image collections and I just saw some stereograph from postbellum Boston, from the collection of the Boston Library. Might I recommend the slideshow? (Go fast; the stereographs are sorted by topic so you're bound to get a whole bunch of very similar images.)
The churches interest me, of course, and fortunately they come early on, after the Masonic temples. None were Universalist, but "Dr. Gannett's church" (Arlington Street), King's Chapel, and the defunct Hollis Street Church (another) and Church of the Disciples are Unitarian churches represented.