Plexamp has a drop down option for source, but it seems meaningless. I have 3 music libraries. One is compressed AAC/MP3, another lossless (so it has duplicates of the 1st library), and a 3rd is my Dad's. When I search, you get results from all 3 libraries, not just the selected source. Nor does the source change if you play a result in a different library. Maybe it's a bug, or maybe I'm not understanding how it should work. I think ideally one should be able to pick multiple sources, but the search should be restricted to those sources.