A Nearby Combini in Japan
Convenience Stores Are Everywhere in Japan
If you are in desperate need of a map or guidebook, look for a nearby convenience store (they are everywhere). You will definitely find one. However, selection is limited and finding an English guidebook or map would be rare.
If you want an English guidebook or map, you will have a better chance visiting one of the larger bookstores in Tokyo like Yurindo. These will have English book sections where you will most likely find English maps and guidebooks.
There is also always the option of purchasing one before your visit to Japan in your home country or at the airport. Your hotel will also have English guidebooks and maps.
The easiest and cheapest (free) option is of course google maps!