I've seen the anime, but not read the manga. I looked through my own search methods and couldn't come up with much, myself, for a map. There are a few Japanese art books related to the series, but I don't know the exact contents. Sometimes such books contain maps, even if they are just rough ones. I saw one listed on EBay, so you could contact the seller to see if it has a world map in it. Just do a EBay search for "Berserk artbook" and it will come up. If you don't know of it already, a site called arcticnightfall.com is devoted to Berserk and has listings of various merchandise that came out for it, including the art books, so there's another avenue to look into them, if you wish. Again, I don't know if these books have any map data in them, but they seem like a possible source.

Other than that, there is a new series of Berserk animated movies currently coming out in Japan, and the DVDs, when/if they are released overseas, may or may not have map info in the scenes or in extras.