On Moorea we booked through the excursion desk, just a couple of days in advance. It was good that we did, since the large corporate group on board was having a rally, and had booked almost all the cars on the island. But that was pretty unusual, and we ended up with the local company.
I think in BB, like Kathy says, there are vehicles right there near the dock.
Note that if you need an automatic, book in advance for sure, they are pretty rare and more expensive.