I think the font LaTeX uses depends on the
documentclass and other settings, but by default, it should be using the Computer Modern family of fonts. These can be downloaded from http://www.ctan.org/tex-archive/fonts/amsfonts/. I think
cmsy-name like fonts contain math symbols,
cmbsy-name like fonts contain bold math symbols,
cmex-name like fonts even more symbols, etc. Just download the fonts and look at them in the font editor. I am no expert for fonts, but I think they are not unicode and all just support a limited set of characters. They are under the SIL Open Fonts Licencse, which should make them free for non-commercial usage (but better check this by yourself and don't quote me on that ^_^). They are not truetype, but pfb, which can be loaded in Java, for instance, but seemingly cannot be opened natively under Windows.
However, unless you need your text to look exactly like LaTeX' output and/or you need to distribute the fonts with your package, I would rather recommend using unicode fonts like, e.g., Arial Unicode. Using tools like
charmap, you can find the right unicode code point for the maths symbol you like and then use that symbol. This way, you can always replace the font with another unicode font supporting the symbols.