Item Issue / Questions about Sourcing Requisitions to Inventory only and not to PO's

We have a situation where we have a group of items that are inventory items but in certain situations we want them to create a PO.  We always want all the others to fill from inventory.  So just for a select few we want them to create a PO for others go straight to inventory.

Example:

Item 1 for dept 1 would get sourced to a PO

Item 1 for dept 2 would get sourced out of inventory.

Item 2 for dept 1 would get sourced from inventory

Item 2 for dept 2 would get sourced from inventory.

Item 1 and 2 are inventory items.  We do not want PO's to get created if we are short of inventory for requisitions submitted by either department