How can we limit the suppliers shown on the requisitions form to only suppliers who have sites assig

We are running into issues on requisisitons with people selecting a supplier, when they don't find a site they just continue with the requisition.  Then when we try to convert to a PO we realize the supplier does not have active sites assigned to that ord.   How can we limit the suppliers shown on the requisition form to only suppliers with active sites for that org?  We have another org who has been good about not selecting a supplier if they don't have a site but they are getting frustrated having to look through all of the suppliers instead of