The most likely place to find a library/artifact is Maven Central. This is the default repository used by Maven builds. See http://maven.apache.org/guides/introduction/introduction-to-repositories.html for more info. However, you can reference multiple repositories in your build.
http://mvnrepository.com is a nice frontend for artifact discovery, but as far as I can tell doesn't host the artifacts, it just provides pointers to them.
Note that Java and Maven aren't synonymous, but they are tightly coupled, so Maven Central isn't the official "authoritative" artifact store, but it is the ad-hoc standard, and is used by other Java-associated dependency management tools like Ivy or Gradle.