A serch on Ebsco Academic Premier for "United States" and "Puerto Rico" provided 1,657 hits. I am not sure which aspect of those two terms you were looking for, but clicking on "subject" and "politics and government" on the right gave me 26 hits. This is a way to narrow a search down. I always select "full text" as a limiter, too. The same search in Credo Reference yielded 1,199 hits under "subject". The search in Facts on File World News Digest resulted in 164 News Digest articles, plus 125 Reference Shelf, 30 Analysis and Background, and 1 Breaking News hits. I did enter the tern=m "United States" instead of US -- sometimes this makes a difference. I hope this helps.