EASY!
Places to go are definitely konbini @ convenience stores. There are lots of them in Japan, which include 7Eleven, Lawsons and Family Mart. I usually buy onigiris here, cause they costs around 100 to 200Yen each, depending on what type you take. As I opt for the seafood-friendly menu, toro (tuna) or salmon is the way to go! I only knew how to speak basic Japanese, so kept repeating the same sentences at every konbini I go. :P
Those phrases are:
1. Igo toro desuka?
2. Igo salmon des?
See, jumbled up Japanese words but they understood anyway. haha.
Another place to buy cheap food or eat for free is the supermarket beneath every departmental store. Their options are great, and if you go there 30 mins before they close for the day, you can find good deals!
A bowl of udon or ramen is also very cheap if you go to their fast food chain, Yoshinoya which costs around 300+ yen. In small restaurants, they are typically priced from 600-1000Yen.
Did i mention that japan has the cheapest evian mineral water in the world?? A 750ml bottle costs a little over 100yen, and it usually cost more than 1euro the last time i bought one in Paris! Super cheap.