I'm not entirely sure, but I think you might find it hard for an artist to sell you his complete source files. You could have a couple layers however. One of the map and another with the names of locations that could be modified.
Full source files would show all the tricks an artist used to create the image and while some might not be bothered by that others might not want to give it all away. There is also an issue of source files not working for you because everyone uses different software. For instance, Paintshop, Gimp, Xara, Inkscape, and various other products which unless you have that software the files would be useless to you.
I would recommend you just ask for a way to modify the labels so you can change them later and let the artist consider how to allow for it. I would think that shouldn't be a problem. Of course, sometimes if you are planning to post it on the net some might be concerned about how you change those names. You would be surprised how much an image can be degraded by poorly placed labels. Some are concerned that if their name is going to be on it that it hasn't been massively butchered. It is their reputation after all just as much as it is your map to use.