Where to find custom beamer themes

All (or at least most) of the default themes are available to view here. If none of these suit, I think the best bet is probably to pick one you almost like and modify it yourself. This has the advantage that you learn more about beamer, and it makes your presentations unique. The beamer manual is pretty complete and all the relevant options are documented...

Customising a theme does not involve hunting out the .sty file in your texmf tree and editing that: you can just modify options piecemeal in your preamble. For example, a recent beamer preamble of mine starts like this:

\documentclass{beamer}
\usetheme{Dresden}
\usecolortheme{beaver}
\setbeamercolor{itemize item}{fg=darkred!80!black}

Note that first I pick some theme options, then (the final line) I redefine the colour of the itemize item. As long as the command comes after the colortheme option, this gives you an easy way to modify a beamer theme on the fly. [I think I did this because the beaver theme is red, but it doesn't actually redefine the itemize colour, so you get the default blue unless you fix it.]

There are some custom themes available here. But I'm sure you know about this because you obviously googled "custom beamer themes" before you asked a question on here.

Some additions that I did not see in the links provided by Seamus:

• The progressbar Beamer theme with a customizable progress bar at the frame bottom

• The Keynote theme

This may be a shameless plug, but I ported a Powerpoint and a Powerdot theme to Beamer. They are available here: https://bitbucket.org/marczellm/beamerports. Here is what they look like:

I would be happy to receive improvement suggestions, feature requests and bug reports.