Saying that you want to retrieve an element called Message_Source without knowing what namespace it's in is a bit like saying you want to go to London but you're not sure whether it's London, Ontario or London, England. It suggests there's something very fishy in the way you are using namespaces.
However, you can do it. In XPath 2.0 you can do select="*:Message_Source", in 1.0 you can do select=*[local-name()='Message_Source]". It may not be especially efficient.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference