Newspaper collections aren't typically organized by "undergroundness" unfortunately. And truly undergound materials don't often make it to today! Have you tried the Proquest collection of historic Black newspapers? It's a popular one, your college is likely to subscribe if they have an American history program. This directory of digitized Black newspapers is also helpful. But any city with a sizeable Black population likely had a Black paper or two. They weren't really underground though. I assume that's what you're getting at with "ethnic," if you were thinking of another group let me know.