They appear in different places different years. They used to happen on the stretch between Mulsanne and Indianapolis but they closed that off very early last year. Roumor has it that they will close off that part for the duration. I've seen them out side Bleu and Houx Annnex. You also get the local Kiddies on Bikes doing Donuts, Wheelies and Endos outside Karting Nord. These things arn't organized they just happen. Just hope your there when they do.
Hope that helps.
t.