Sometimes your default supplier is not the supplier you'd like to send an order to, and you need to change the supplier on items that are already on an order. On the order, simply click on a SKU that you want to change suppliers, and it will pull up the item details page. Click "Show all suppliers" and find the supplier you want to order from. Click "Add to Order" for that supplier, and add it to the PO you're working on.
Don't forget click "Remove" on the item from the supplier that you don't want to order from.
Do this for each item that you need to change suppliers until they are all listed under the correct supplier for the order. If the supplier you chose should always be the supplier of choice for those items, you can change your default supplier manually for next time: Changing Your Default Supplier.