If you've never been to Japan, you'd be amazed at the prevalence of convenience stores in major cities across the country. There seems to be a turf war going on between the big names, and unless you are surrounded by cows, or on a desolate island, it's practically impossible to not spot a "konbini" in Japan. What I'm trying to say is, it is everywhere, and having it everywhere is very, urrr, convenient.
Major "Konbini" Stores In Japan
Japanese people like to shorten everything so "convenience store" is always referred to as "konbini" in Japan. There are three top konbini chains and they are "Seven Eleven", "Lawson", and "Family Mart". There are slight discrepancies in what each store carries depending on the location and the size of the shop, but you can pretty much get your basic needs fulfilled going to any one of them.
Gluten-Free Items at Convenience Store
So here are a few items that I can buy from konbini which are likely to be gluten free. Please note, most items share their production lines with wheat, so if you can't take the risk of possible cross-contamination, please steer clear of the items below unless noted otherwise.
My emergency ration and also one of my favorite munchery, the Soyjoy. It is made out of soy (who would've guessed) and is gluten free according to their website. It appears they have twelve different flavors, although most stores I've been to only carry a handful.
My favorites are White Macadamia, Plain, and Hawthorn Berry.
When you are on the go, but nothing gluten-free in sight, these fellas can come to your rescue from a nearby konbini.
Rice Balls (Onigiri)
Rice balls are big in Japan. Every konbini sells rice balls and if it doesn't, it's an alternative konbini, or rice balls simply flew off the shelf for that day. Since soy sauce is the main flavoring agent in Japanese cooking, most rice balls unfortunately are not celiac-friendly, but no need to despair. I found three flavors that tend to not have any gluten contents, and they are "Salt（塩)", "Salmon（鮭）", and "Pickled Plum（梅）".
Remember, Japan is the birthplace for MSG, and some onigiri contain the chemical and other preservatives. But if it doesn't bother you, or you are willing to let it slide during your stay in Japan, onigiri can be your best friend.
Not all konbini stores sell produce but most sell bananas. Unless you are allergic to banana or you loathe it, it can give you the boost you need. If you are used to shopping at Trader Joe's, a popular American grocery chain only available in few states in the US, you'd be amazed at the price of one banana, which is usually around 100 yen, as supposed to 20 cents in the States. But it's Japan and things are expensive here.
I always buy a boiled egg to go with my onigiri for lunch from konbini. The ingredients are really simple, just egg and salt usually. Just make sure you get the right kind（味付けたまご）; they also sell half-boiled eggs (温泉たまご）right next to it which is not meant to be eaten like a boiled egg.
Snacks and Sweets
Most potato chips and fried veggies tend to have no wheat contents. I'm allergic to chocolate and peanuts so I can't vouch for it, but I'm guessing most chocolates are gluten free too (don't quote me on it though.)
Snacks and sweets are a bit tricky because many Japanese sweets contain maltose, and maltose is that gray area where I'm not quite sure if it's gluten free. So it might be safe just to stay away from the snacks and sweets department unless you are absolutely sure it is gluten free. Well, at least soyjoy and bananas are sweet, right?
Gluten Free Card
So I've seen this thing called "Gluten Free Card" on the internet for celiac patients to carry and to use at restaurants. Some cards I've seen are just plain wrong, in terms of grammar, and some are just way too long to use at a konbini. People go to konbini to get things yesterday, so they probably won't have all day to read an essay.
So here is the abbreviated version of gluten free card that I made that you can use at a konbini.
Japanese food products rarely contain rye, or barley (drinks are different so watch out). Wheat is the only gluten product that needs to be outlined in the ingredient label in Japan so if a product contains any wheat, it should say so on the package, and store clerks should be able to help you identify it.
Please feel free to use it if you'd like. Bon Apetit!