I'm not sure I understand your problem....
But it feels like you're trying to hack through the UI hierarchy to find something (ala WinForms) - this is like swimming against the tide in WPF. It'll make you tired real fast..
Look around for articles on the ViewModel pattern. One of them that I read is one by Dan Crevier. The basic idea is to have a Model object for the UI - the UI (data)binds to properties exposed by the ViewModel. Now all your UI related logic could go into the ViewModel, which is notified via propertyChanged notifications of any change in session state. So if a field changes, the ViewModel gets notified. The ViewModel updates relevant properties and the UI refreshes accordingly.
In this case ViewModel.OrderInfo.Find_PO_with_X(searchCriteria) will help you get to the right object. If found, you say ViewModel.ShowGreenLight = true and the UI control bound to this property will turn green.
HTH