One of the best sources of channel icons is http://www.lyngsat-logo.com/ . There, you can browse the available icons and choose those appropriate for your system. However, doing so can be a lot of work (requiring many page hits and a lot of time).
An easier approach is to use some new features in the post-0.20-SVN branch of MythTV to quickly look up the information. And, the best part is that you can use this new feature even if you're not using SVN--there's no need to upgrade any part of your MythTV installation.
First, checkout the appropriate portion of the SVN branch with:
svn co http://svn.mythtv.org/svn/trunk/mythtv/contrib/master_iconmap
Doing so will create a directory called "master_iconmap" underneath the current directory. The master_iconmap directory contains a file called "channel_icons.pl", which can be used to retrieve channel icon information. See http://www.gossamer-threads.com/lists/mythtv/commits/255099#255099 and "./master_iconmap/channel_icons.pl --usage" for more information.