It appears the OP leaves in 2 days for her trip, so probably not time for mail order.
OP, I could be completely wrong, but I doubt the cards you'd buy at the parks will be class 6 or higher. If that is important to you, then you need to pick one up before you go. As high as that seems compared to the sales you can get online, I guarantee it will be a far better deal than what you'll get in the park. I tend to pick up a card or 2 before every trip. Over time, I've put together a decent collection and finally think I can do a 10 day trip shooting RAW with no computer. I enjoy shooting with abandon, not worried at all about running out of space.