"Most do it on Motorbike"
Dunno about that. You count the number of cars compared to bikes? Without experience it's not the sort of thing a person should be doing, riding a bike around mountain areas. Car is much safer and better if it rains.
You can buy a map in 7/11s if the car hire place doesn't have a good one, otherwise look online and print one out. Northern Wheels is a good place to hire a car.
There's a list of map sales outlets on the GT-Rider website
I sell them too, in case you have trouble locating.
Given how rainy it's been lately I'd definitely choose a car over a bike. Check out http://www.northwheels.com