[nat]

Find a good book and go through the bibliography. If you know that it's a high-quality book you know that you're going to be able to trust their sources. The only real problem is that this method can only take you back in time.

JSTOR is wonderful; don't know how useful it is for baseball or how expensive it is (my access is through my employer), but back issues of gazillions of journals is great to have at your fingertips.

If you find an academic article that you want to read but can't get, you can try e-mailing the author. They'll probably send you a pdf.

Also - look up a book in the library that you think will be useful to you, and then look around in the stacks for other books near it. Doesn't always work (cataloguing seems to be a mysterious and dark art to me), and who knows if they've got anything else that would be helpful, but sometimes you'll discover books that you didn't know about this way.