Old, original maps of Africa, like the same maps of elsewhere, tend to be quite expensive, as some have already pointed out. But there is often another way to get your hands on original maps which is often cheaper.
Many of the books published by explorers of the 18th, 19th and early 20th centuries contained folding maps of their travels, some bound in but folding out, but many loose and inserted into a slot in the back cover. Examples include almost all the books by Henry Stanley - Through the Dark Continent, In Darkest Africa, etc., but also many, many others. The first editions of these books tend to be expensive, but second and subsequent editions are usually much cheaper and often include the same maps. Oddly, I’ve seen maps originally included with some of these books for sale for more than the books with the maps. I assume the maps appeal to a different audience, and by separating them from the original book someone can get more money than selling them together. The book in the
@rigbymauser post, above, is an example.