Buying an IPhone in Nigeria.

You could talk to friends with iphones to see what kind of after sales service they enjoy, or you can try buying from a network operator monthly if you will not change operators in the near future.

Or you can reach out to Apple Support directly. Depending on your region, you may be able to reach them here: contact Apple Support 

https://www.apple.com/ng/

Select the phone you want and work through it as though you were going to buy it. You will finish up with the purchase price and you may or may not get a pay monthly price. If not then you can't.

It doesn't look like it when I went through it.