1-Go to http://www.bullseyecrosshairs.com/
and download a crosshair from the "Quake 3 section".
2-Make sure it is a "pak9" file.
3-Stick it in your base folder.
4-Drop down console and type \cg_drawcrosshair 9
*If you don't like the size use \cg_crosshairsize <your value here> to adjust it (default is around 24 I think).
*Remember these, like skins are for non-pure servers.
[Edit] Just noticed that the only hairs that show up are the ones that completely fill in the square gray box like this one
Most of the translucent ones and round ones don't seem to work.
(bright green one ^)