There are two ways to issue a purchase order to buy parts for your shop.

To purchase parts for an open repair order, when you are in that repair order click the Issue PO button on the left-hand side. This will allow you to issue a purchase order for all the parts that you've added to the repair order.

To issue a purchase order to purchase parts for inventory or supplies for the shop, from the work-in-process go to inventory. In inventory you could now issue a PO to each vendor for the parts they supply.
 

Purchase Orders Overview Video