The United States Copyright Office. You can do it yourself.
The Library of Congress page for publishers. Lots of helpful info, including how to register with The Library of Congress.
The Absolute Write forums. Whenever you search for an agent or editor, you’ll get at least one hit off these forums. You might have to wade through some off-topic comments, but this is usually a great place to find info from people who’ve queried and researched said agents or editors.
Well, that’s all I’ve got for now. I’ll try to think of some others later.