These and these are both good options. In either case you will need to paint/sharpee over the modern logos, particularly for Winterfell. They have good garb standards.

As for general advice- Don't drink to much and don't talk shit if you can't back it up. Pack a set of dry modern clothes in a waterproof container and more socks and underwear than you think you'll need. Make sure to get enough water, particularly if you are drinking. Participate in as many things as you can. The monster hunt is awesome. Do it. Get some pickup fights in. Attend court. All shit worth doing. Hivemind, who runs the thing, posts on reddit, so I'm sure he'll pop up at some point. This is going to be a great first event.

Do you have a group you are rolling with? What do you have in terms of weapons?