Nigerian stamps? Are you sure you're not getting them mixed up with the now worthless zimbabwean dollars? Never heard of Nigerian stamps before.
This website sells them, and it's one of the largest stamp companies in the world.
http://www.sandafayre.com/countries/nigerianstamps.htmlThe Nigerian stamps on the first page cost between £50 and £800. Zimbabwean dollars are now worthless, but Nigerian stamps are a good investment from the looks of those prices.