You can buy them at both parks at the ticket windows in front of the main gates.
The ticket lines at 08:30am are short as the people start to arrive.
Your
HHN tickets lets you stay inside. They call it a Stay and Scream ticket, but it's just your HHN ticket.
At 10am to noon, the lines outside to buy tickets are huge. It boggles my mind the amount of people who show up late. You're going right before Halloween. Tuff to say if those days will sell out ahead of time.
If you do decide to wait until the day you go, arrive to a park early and buy your HHN ticket at 08:30am. Don't wait until the afternoon, the lines will be long. Or you can do like Barry suggested and buy it online, then pick it up at the Kiosk machines which are located next to the ticket windows.