Well, you can buy GA tickets at the circuit so no need to worry about those just yet, plenty of guys and girls will give you advice on campsites on here, and if you are very lucky they may have a spare ticket, if not it's the agencies or fleebay I'm afraid.
Welcome to CA btw, and I doubt that this will be your last trip to LM. I did a couple of organised trips with agencies back in the 80's, but I soon realised doing it your own way was the best.