Where can I find the symbol for …

There is a wide range of symbols available for use with TeX, most of which are not shown (or even mentioned) in (La)TeX books. The Comprehensive LaTeX Symbol List (by Scott Pakin et al.) illustrates over 2000 symbols, and details the commands and the LaTeX packages needed to produce them.

Other questions in this FAQ offer specific help on kinds of symbols:

A useful new symbol search web site is available. It provides you a scratch area on which you draw the symbol you’re thinking of, with your mouse; when you’ve finished drawing, the classifier tries to match your sketch with symbols it knows about. The matching process is pretty good, even for the sketches of a really poor draughtsman, and it’s often worth trying more than once. Given the huge size of the symbol list, this tool can be very helpful.

Symbol List
Browse info/symbols/comprehensive/; there are processed versions in both PostScript and PDF forms for both A4 and letter paper.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=symbols