Testa detta:
SourceLabel = CType(PreviousPage.FindControlByID(PreviousPage.Controls,"lblMyId"), Label)
Public Shared Function FindControlByID(ByVal controls As ControlCollection, ByVal id As String) As Control
Dim found As Control = Nothing
For Each control As Control In controls
If control.ID = id Then
Return control
End If
If control.HasControls() Then
found = FindControlByID(control.Controls, id)
If found IsNot Nothing Then
Return found
End If
End If
Next
Return found
End Function