we purchased the National Geographic Adventure Travel Map from Amazon
It is nice that its a heavy duty, water resistant map. It seems to be pretty accurate and robust. Main only complaint is that it splits the island in half... one half per side. There is a few inches overlap on each side, but it was somewhat annoying to not see the whole map at once during the travel planning phase.
*Edit - Sorry, I just read your post again and realized I missed something key. I don't know how accurate it is for going far off of the Ring Road.