This is of course no complete list, but things that helped me so far:
JSTOR - collects articles in journals and also has some books. The access is limited, though. I think you either need to pay or enter the page via a university. I checked, and they don't have the paper.
PhilPapers - a comprehensive directory of online philosophical articles and books by academic philosophers. Your paper is listed there, but not downloadable.
Libraries - My university has the book. If yours hasn't, you can still order it and they will send it to you.
The homepage of the author - in this case there is not even a publication list, but many authors put .pdf's of their articles in their profiles (e.g. Sider, to name but one).
A site that help me out sometimes is academia.edu: They also allow sharing of articles, but again, yours isn't there.