Create a PO from a BOM

This must be possible and I must be missing something simple.

I want to create purchase orders to bring items into stock in order to build my assembly item. I do not have a sales order, I just want to build some assembly items to have them in stock to ship out when I do receive a sales order

This is what I am doing:

I create my items - I create my assembly item - I create a BOM stating how many items I need to build on assembly item - NS tells me that do not have enough items in stock to build my assembly item - Yes, I know that. I want NS to create purchase orders and bring the items into stock. I can see that my BOM as a 'Back Ordered' quantity - but I cannot see how t raise the PO