Lots of info here:
http://www.club-arnage.com
The short answer is that your only hope for (camping) tickets for the 2005 event is from the big agencies like Page&Moy, Just Tickets, Airtrack etc.
Car park and admission tickets can be bought 'on the door' at the circuit.
Official price list (to compare with Agency Top Dollar prices) at
http://www.lemans.org