Flight booking in Mada

Hi and thanks to everyone for all yuor previous advice on my trip to Madagascar. i think we are almost there now, I just have one more question...

We were going to book our domestice flights online through the Air Madagascar website however I've just seen some posts on the forum that suggest it is cheaper to book the flights in Madagagascar. Our guide book (Bradt) says that flights can get full so we're slightly worried about leaving it until we get there. Does anyone know whether we will be ok to book flights Tana - Morondava, Morondava - Antisiranana (via Tana), Nosy Be - Tana with a few days notice?