The most interesting places to get snacks in Japan are at dagashi-ya, which means, literally cheap snack store. Dagashi are mostly very inexpensive candies and other snack items that have been around for many years, especially in the mid-Showa post-war period. While the prices have gone up since the 50s, there are still many dagashi that you can buy for under 50 yen. Many towns have dagashi-ya, so ask around. There is at least one dagashi-ya in Akihabara, and there are several in both Shibuya and Shinjuku, so no matter where you are in Tokyo (or Japan), you can find dagashi. Enjoy!!