Where can I find examples of decorated borders/margins?

You could take a look to niceframe and umrand (Examples see below)

umrand: in http://mirrors.ctan.org/macros/generic/umrand/umrand12.zip is a dvi-file with examples. See below (It's the German description, but it contains a lot of examples).

niceframe:

\documentclass[12pt]{scrartcl}
\usepackage{blindtext}
\usepackage[utf8]{inputenc}
\usepackage{niceframe}
\usepackage{swrule}% englische Linien
% ----------------------------------------------------------------
\begin{document}


\section{niceframe}

\subsection{niceframe}
\niceframe{\blindtext}

\subsection{artdecoframe}
\artdecoframe{\blindtext}

\subsection{\curlyframe}
\curlyframe{\blindtext}


~\swrule{0.6\textwidth}{1pt}~


\end{document}

You may want to have a look at vectorian.