X mark to match checkmark

I would think your best bet might come from pifont's dingbats:

\documentclass{article}
\usepackage{amssymb}% http://ctan.org/pkg/amssymb
\usepackage{pifont}% http://ctan.org/pkg/pifont
\newcommand{\cmark}{\ding{51}}%
\newcommand{\xmark}{\ding{55}}%
\begin{document}
\verb|\checkmark|: \checkmark \par
\verb|\cmark|: \cmark \par
\verb|\xmark|: \xmark
\end{document}

\cmark is defined as \ding{51}, while \xmark is defined as \ding{55}. Here is a list of the dingbat symbols, taken from the pifont quick reference guide:

If using XeLaTex is an option, then you could use one of Unicode's X mark (cf. https://en.wikipedia.org/wiki/X_mark for a complete list). All characters may not be available in your favorite font, but, as noted in this answer, DejaVu Sans has a pretty good support.

On my installation, when I compile the following with XeLaTeX,

% !TeX TXS-program:compile = txs:///xelatex/
\documentclass{article}
\usepackage{xltxtra,xunicode}
\setmainfont{DejaVu Sans}
\begin{document}
    \begin{tabular}{c c c}
        Symbol & Unicode Codepoint (Hex) & Name\\
        ☐ & U+2610 & BALLOT BOX (checkbox)\\
        ☒ & U+2612 & BALLOT BOX WITH X (square with cross)\\
        ✗ & U+2717 & BALLOT X (cross)\\
        ✘ & U+2718 & HEAVY BALLOT X (bold cross)\\
        × & U+00D7 & MULTIPLICATION SIGN (z notation Cartesian product)\\
        ╳ & U+2573 & BOX DRAWINGS LIGHT DIAGONAL CROSS\\
        ☓ & U+2613 & SALTIRE (St Andrew's Cross)\\
        ✕ & U+2715 & MULTIPLICATION X\\
        ✖ & U+2716 & HEAVY MULTIPLICATION X\\
        ❌ & U+274C & CROSS MARK\\
        ❎ & U+274E & NEGATIVE SQUARED CROSS MARK\\
        ⨉ & U+2A09 & N-ARY TIMES OPERATOR\\
        ⨯ & U+2A2F & VECTOR OR CROSS PRODUCT\\
         & U+1F5D9 & CANCELLATION X\\
         & U+1F5F4 & BALLOT SCRIPT X\\
         & U+1F7A9 & LIGHT SALTIRE
    \end{tabular}
\end{document}

I obtain:

Note that it is possible to set the font to DejaVu Sans only for those symbols: again, follow the explanations at https://tex.stackexchange.com/a/291955/34551.