Zoomarine operate regular buses from Lagos to Park. A popular departure point is the Bus stop opposite Pingo Doce near the Marina.
There is a Zoomarine kiosk on the main concourse of Faro Airport where you could buy tickets on arrival.
The zoomarine website (www.zoomarine.com) states that you do not need to buy tickets in advance. You can purchase on arrival.
There are a lot of shops in Lagos who are agents for Zoomarine.
I think from memory that the Bom Dia shop in the Marina sells them.
