Theirry has some great sign meshes that have editable textures: http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=8174.msg196067#msg196067 (http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=8174.msg196067#msg196067)
You can edit the sign texture in just about any program, such as paint, just by saving it afterwards as the original name. c: