You can get the Yellow Fever vaccine and card at all international borders or Port Health offices in Nigeria. In Lagos, the Port Health office is located at Airport Rd, right by the toll gate on your way to the International airport. Look out for a yellow coloured building to your left (before the toll) if you’re coming from Ikeja. Apart from the Port Health office at Ikeja, you can get the vaccine and Yellow Card at Tincan Island Sea Port, Apapa Sea Port, Idiroko border, Seme border and all Federal Secretariats. If you are planning to travel via the land borders (especially those in Lagos), I would advise that you get your card before your trip, rather than getting it at the border. The land borders in Lagos are notorious for extortion, best not to give them a reason to extort you further.