If you haven't booked up already, you're going to struggle to get anything at this point.
Some places have a handful of camping sites left, but they won't be the better ones. Saying that, as long as you have somewhere to sleep, then that's all that matters really! Tickets for the main event can be bought on the gate, but for ease you might as well just pay the extra and get them from your operator. I'd definitely get a grandstand, that way you're guaranteed the sun is going to shine all weekend whereas if you didn't then you know it's going to rain Le Mans is one of those events that's different for everyone, and there's no right or wrong way to enjoy it.
And I'll second the 'don't speed' thing. Having had my licence confiscated and relieved of 750EU at a cash point (never mind them chasing me for another 750EU when I got home), I can confirm they don't much around. That said, you can have a laugh with the gendarmes when they're trying to fill the paperwork out and you don't speak French: You try miming 'plumber'...!