Your Musicnotes order number will be contained in the Order Confirmation Email that is sent to you shortly after placing your order.
Or, you may click My Account at the top of any page, and enter your account information if you aren't already signed in. Under the ‘Order History’ heading, click View Past Orders.
From there, the Order History page will be sorted with your most recent order first, and it will be expanded. You can find your order by date, and either click the arrow to the left of the row, or you can click the "Expand All" link at the top of the Order History page.
You can also access your receipts (which list order number) by clicking here.