If an item does not have a cost loaded for your supplier, or if you have a special price on a product from a supplier, you can manually set a custom cost on an item.
To do this, navigate to the product details page. On that page, click the cost from your supplier, and you can edit it. You'll then have the option to add the item to an order.
Please note that if you have Managed Pricing enabled for the vendor, your manual cost will override any future price changes from the supplier for your price tier.