USA Open Tickets
Official ticket sales are the best option, available through official ticket outlets. See the official website for more details.
Any search online for "USA Open Tickets' will come up with thousands of websites selling tickets, and there is no sure way of knowing which of these resellers are reputable. If you were unable to get a ticket through official sources, and you are desperate and willing to spend a lot of money, these may be the only option available to you.