There are numerous threads on here about forumophilia invitations. You will, however, most likely not get one for free.

Just be patient. They're opening their registration every other week. I think people here always announce it so you won't miss it, so why pay somebody for an invitation code when you can get it for free. A hint: When they actually do open registration, make sure to register at least 2 or 3 accounts so if you should get banned (and chances are that you will if you didn't know of this board yet), you still have a backup account just in case registration is closed again.