You can simply put ar.sty
package in your working directory. Here is it's code:
%
% This package is for using the aspect ratio symbol in math mode and in
% text mode by means of the command \AR.
%
% It is supposed to work both with LaTeX209 and LaTeXe
% It requires the METAFONT files ar5.mf, ar6.mf, ar7.mf, ar8.mf, ar9.mf,
% ar10.mf, ar12.mf
%
\ifx\documentclass\undefined
% With LaTeX209 the symbol is available in just one size (by default)
% but the user can edit the next line in order to get the size s/he wants
\newfont{\ar}{ar10 at 10pt}
\load{\normalsize}{\ar}
\def\AR{\mbox{\ar A}}
\else
% While with LaTeXe all standard sizes are available
% (no bold version available...sorry!)
\DeclareFontFamily{U}{ar}{}
\DeclareFontShape{U}{ar}{m}{it}%
{<5><6><7><8><9>gen*ar%
<10><10.95>ar10%
<12><14.4><17.28><20.74><24.88>ar12%
}{}
\DeclareFontShape{U}{ar}{b}{it}%
{<->ssub*ar/m/it}{}
\DeclareFontShape{U}{ar}{bx}{it}%
{<->ssub*ar/m/it}{}
\DeclareMathAlphabet{\mar}{U}{ar}{m}{it}%
\SetMathAlphabet{\mar}{normal}{U}{ar}{m}{it}%
\SetMathAlphabet{\mar}{bold}{U}{ar}{bx}{it}% actually uses medium series
\if@compatibility
\def\tar{\fontencoding{U}\fontfamily{ar}\itshape}
\DeclareRobustCommand{\ar}{\ifmmode\let\cf@ar\mar\else\let\cf@ar\tar\fi\cf@ar}
\else
\DeclareOldFontCommand{\ar}{\fontencoding{U}\fontfamily{ar}\itshape}{\mar}%
\fi
\DeclareRobustCommand{\AR}{{\ar A}}
\fi
\endinput
Then you have to make sure that you have at least the ar10.mf
file in the same directory.
And if you want all the sizes, use the files from this repository.
texlive-fonts-extra
package, using synaptic or apt-get.