Hi,
Simple PO https://apps.shopify.com/simple-po is exactly what you are looking for (in my opinion anyways!)
Simple PO allows you to create and link suppliers and cost of goods for your products and then you can generate a purchase order from an Order, Product or Supplier.
If you're a dropshipper you can quickly email your vendor an order with 3 clicks once you're set up or if you hold your own stock you can order more of one or more products your vendor makes, again all of the addresses and items are filled out for you and you can select whether the shipping address is your own or the customer's.
There's a 7 day free trial so give it a go today, any other questions please don't hesitate to ask.
Thanks
GhostApps