As AJed mentioned, ACM digital library and IEEE xplorer would be near the top of list.
Additionally Goggling with advanced search for the name of the reference with the option to search for PDF or PS increases the change of a hit.
Sometimes limiting the search to
xyz.edu increases the quality of results.
If you are just starting out on a subject, try adding the word
survey as a search term because many researchers write survey papers so that someone new to the subject has an informed person guiding the way.
I also like to hit CitSeerX, WorldCat and Google Scholar
Don't forget to look at the patents in Google Scholar, I don't usually but every now and then companies have to give out something good they may not publish anywhere else.
If you don't attend school, you can see if a school provides access to their library for outside third parties. I found out that I can do this for Princeton University for $300 a year.
Microsoft Academic Search
This can also visualize publication trends in computer
arxiv.org - Open access to 1,106,143 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics.
This has a nice feature that list survey papers which is useful when you are starting on something new.