I guess the ticket office will be at the same adress as the accreditation centre :
CENTRE ACCREDITATION:
ACTISUD
13, Rue Maurice Trintignant
72000 LE MANS
I've been there 2 years ago to see if they still had grandstand tickets for sale, as I used to do in the rotonde. Ticket office was there too back then.
Another tip for the CA guide : during raceweek you can buy tickets in the shop at the museum. For those who are last minute looking for grandstand tickets : there's a good chance you'll be able to get grandstand over there.