I want to get this M in LaTeX:

I don't get this style using \mathcal{M}, \mathbb{M} nor \mathfrak{M}.
How can I get it? I don't want to change the font of the whole document, I just want to insert that M with that style.
I just found it by trial & error with a huge portion of luck. This shape of an M is obviously included in the Euler Virtual Fonts (eulervm):
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{eulervm}
\begin{document}
$\mathcal{M}$
\end{document}

eulervm is designed to be loaded with other fonts not containing math characters. So simply load your other font package.
Another option is to import the letter you need; you can take it, for example, from Euler Script:
\documentclass{article}
\DeclareFontFamily{U}{eus}{\skewchar\font'60}
\DeclareFontShape{U}{eus}{m}{n}{%
<-6>eusm5%
<6-8>eusm7%
<8->eusm10%
}{}
\DeclareFontShape{U}{eus}{b}{n}{%
<-6>eusb5%
<6-8>eusb7%
<8->eusb10%
}{}
\DeclareSymbolFont{eus}{U}{eus}{m}{n}
\DeclareMathSymbol{\Mscr}{2}{eus}{77}
\begin{document}
$\Mscr$
\end{document}

\mathscr{M}works. – Speravir Feb 18 '14 at 03:54