To show only suppliers mapped to the entity raising the PO in the New PO Request page, map suppliers to entities in:
SupplierHub edit the supplier in the Supplier List and goto Entity - Supplier - Item Mapping
SetupApps SupplierHub select Entity Allocation
When you add a new supplier in the New PO Request menu, it is mapped to that particular entity raising that PO only.
When you add a new supplier in SupplierHub it is mapped to all entities.
To unmap, goto Entity Allocation under SupplierHub in SetupApps.