If you have a font containing the symbol you can define a userdefined symbol.

Follow these steps:

Open the Symbols dialog (Tools-Catalog...).
Click 'Edit'.
Type a name for the new symbol in the combobox 'Symbol'.
Type a new name or select an existing name in the combobox 'Symbol set. This is the symbol set you want your symbol to appear in.
Select the font in the listbox 'Font', for example the OpenSysbol font.
Select the formatting (normal, bold, bold-italic or italic) in the listbox 'Typeface'.
Click on the symbol in the table.
Click 'Add' to add your symbol.

You can now use your symbol like any other symbol by selecting it in the Symbols dialog or entering %thenameofthesymbol directly.

StarMath Truetype font set can be found on the web via one of the search engines. Prevenance is unknown but it is useful.

If you don't have the symbol try looking in the StarMath font first; it contains about 25 additional symbols.

Another source of symbols is the so-called AMS fonts of the American Mathematical society.

You can download these in Postscript type 1 format from http://www.ams.org/tex/amsfonts.html.

If you use Microsoft Windows you might prefer to download the unofficial BaKoMa Truetype versions from http://www.ctan.org/tex-archive/fonts/cm/ps-type1/bakoma/.

Refer to the documentation distributed with the fonts for conditions of use.