Where can I find a good index for Rashi? Either online (preferably) or a sefer....
A solution is to use the Chabad online Rashi commentary to fuel Google searches. First, type
and then add your search terms after typing a space. Example:
site:http://www.chabad.org/library/bible_cdo/aid/ אחרון אחרון חביב
to find a Rashi from this week's parsha.
You can search hundreds of books and dozens of commentaries on the Bar Ilan Responsa program.