I see many trips advertised online. Should I book this way or can I wait til I get to Maun?
I really wish that I knew how long you were planning to stay in Botswana. I would urge you to book in advance.
If you had followed the threads Africanman you will notice this is the problem for the OP self camping, self driving and not pre booked in advance.To focus on the question CT you can do the bookings when in Maun but why waste time. Book those best suited to your budget and preferences before you arrive.
It's best to book in advance. Botswana's peak season is May til October and availability is usually tricky around that time. The tour companies usually have trips booked ahead for the coming year. If you plan to book when you are in Maun, at least research two or three companies that you can approach when you are here, just to avoid disappointment.
Book in advance!! No reason to wait. This is peak season so it will be busy.