There was thread on Le Mans betting a bit a go (If memory serves...). The results were inconclusive. William Hill (online) do F1, NASCAR, WRC and CART. Not sure about Le Mans. Apparently it is illegal in France, so there won't be any bookies at the ciruit itself.
I think (so it this is probably wrong!) bookies are meant to give you odds on anything if you go in and ask. Although this doesn't stop them giving you bad odds!