I don't know about other parties selling the annual passes, they are often limited to people who live in France, but if you buy a one day ticket at the entrance and then go to the Bureau Passeport Annuel the money you already paid at the entrance will be deducted from your annual pass price. Also, there are annual passport holders on this site who are happy to meet up with you at the Bureau Passeport Annuel to give you a 10 % discount (they get to collect stamps to renew their pass for free). You can find them in the Sponsorship of Annual Passport thread. Just see if anyone is going to be there on your first day, or post your own message asking for a sponsor.