planning our Le Mans trip and my concern is if I should order parking (Blue) ticket beforehand or are the (non-numbered) parking tickets usually still available on-site on Friday afternoon?
I would hate to order one from ACO just to find out that I will not get it in time before leaving to Le Mans, so I would prefer to do this 'face-to-face'... I believe the dead-line for any ticket orders from ACO was somewhere around early/mid May?
Well as I have (hopefully!) a camping permit I don't have personal experience of the parking arrangements. However I believe that you just turn up, pay on the day, are given a parking permit and you can then leave and re-enter the car park at will.
If you want a camping ticket its slightly different and more challenging. A number of people do camp in the car parks and every year there are stories in advance that the ACO will clamp down on it.