There’s a really useful document (a PDF) on the Internet, which has the codes for loads and loads of symbols you could ever possibly want to use, and probably more. It comes in A4 and letter size.
That document is incredibly long and it’s not always easy to find the symbol you want. Luckily, Detexify^2 – LaTeX symbol classifier is on hand to help. All you do is draw the symbol you want and select it from the list of possible symbols it comes up with! You can help train it, as well, to make it better for everyone! [ ]