The best option is as many have pointed out - Google Scholar.
But the very next I have personally found is ssrn.com - which is an open source repository.
You will also find that more and more papers are being hosted on arXiv.org.
If I am reading a book that links to specific academic papers, I'll first try google scholar, then a google search for the primary author. If the first doesn't return a link to a free copy, you will usually find it on the authors .edu homepage.
But the very next I have personally found is ssrn.com - which is an open source repository.
You will also find that more and more papers are being hosted on arXiv.org.
If I am reading a book that links to specific academic papers, I'll first try google scholar, then a google search for the primary author. If the first doesn't return a link to a free copy, you will usually find it on the authors .edu homepage.