I have a friend currently living in Yaounde.
We would like to know where can you buy them?
At the postal service in central town of Yaounde. But Cameroon government is now mostly using but electronical postal stamp because many agent used to make false postal stamp.
However when you are in Yaounde, take any taxi and say "Poste Centrale" (French prononciation) and go at the central post office of Yaounde to know if it is available.
I will pass the information to my friend.