Looking for the Social Security Death Index? Good news! It is FREE in many places. Some are more difficult to use than others.
The best one I have found is here http://ssdi.rootsweb.ancestry.com/cgi-bin/ssdi.cgi -- this is the advanced search option under rootsweb. It allows you to put in whatever you know. For example: If you know the person's first name is Linda, but you don't know her last name AND you know she was born on May 1, 1900, you can just enter that. Give it a try! It's MUCH more flexible than what is available on many of the other web sites.