Hi, my app Simple Purchase Orders https://apps.shopify.com/simple-po was designed exactly for this purpose.
Simple PO allows you to create suppliers for your products and store cost prices for them. You can then generate a purchase order from an order which is sent to your supplier. It easily handles orders with items from more than one supplier.
If you're a dropshipper you can quickly email your vendor an order with 3 clicks once you're set up all of the addresses and items are automatically imported.
Any other questions please don't hesitate to ask.