This procedure will print the SourceIndex of all the elements in the ActiveDocument.
The SourceIndex is not zero based, so it will be 1 integer larger than the zero based item number.
Sub SourceIndex()
Dim objElement As IHTMLElement
For Each objElement In ActiveDocument.all
Debug.Print objElement.tagName & _
" has source number of " & _
objElement.SourceIndex
Next
End Sub