To set up a product code for a non-stock product, the inventory checkbox should be unchecked.

Add the non-stock to an order sourced to Purchase.

To procure the non-stock product, click on Procurement from the top right dropdown menu and follow through with creating the PO.  More information available here... Purchase Order Sourced from a Work Order.

You can create a non-stock product for special orders from a particular supplier.  You can add the supplier on the product set up screen, so that it will be filled in when creating the purchase order.  This will keep all special order items grouped together in the product history.  The special order product can also be added to a category for reporting and lookup.