If I could make a suggestion for your post, I am a novelist myself and was asking for the same thing on this forum. I find for these kinds of requests it would be nice for the cartographer to offer a small margin of profit from your novel if you use the image in the final publication. I used a "Paid?" tag for my post and I got responses very quickly. I think if you use the tag and make it very clear early on that they will get full recognition for the creation of the map and a small profit for creating the map for the novel, I think you will get more interest. I understand that the concept of the map is yours alone, but even the cover illustrator gets recognition and profit from a published book, so I think a cartographer would feel it only fair to be offered a cut of the profit for the book.
Good luck in all you do and I hope to read your novel someday!

~Rana