I get mine through the US, one of the guys in the US office comes back every quarter and I get him to order them through an online health and sports company as they are seriously cheap. Love the Coconut ones!
In US, I carried a Powerbar or Cliffbar or two all of the time because lack of food stops. In Japan, I've stopped carrying because of combinii's.
On a side note, a Powerbar saved my spine a few years back. I was struck by a car doing about 60kph from behind while I was stopped at a red light on my bike (sitting on my top tube) in California. The bike, seatpost, and seat rails were titanium, and the seat and post struck my back. Seat post was bent, and seat itself was toast, but my back was fine - thanks to a powerbar that absorbed/spread impact.
The Weider Energy gel packs that you can buy at konbinis and drug stores are pretty good. The cheapest price I've found for them is 145 yen per pack at a drug store; they're about 200 yen in the konbinis. (But if you're at a konbini, an onigiri would be cheaper .)