You can sometimes book - if they have a telephone number or email address - either directly or through the local Mairie, but most people don't. They do sometimes get full - so the motto is to get there early if you're travelling in peak season. If you plan to arrive about 4.00 pm then you'll beat most other people - the mad rush won't start until about 5.00 through until 6.30 or 7.00 ish.
Many sites have barriers, now, though, so check out the barrier opening times. We've been on a couple of sites where they are actually padlocked in place so if you want an early start best to find out before you get to the gate and find it won't open!