If it's just a snack you're looking for, some more ideas are in Top Ten Low-Carb Foods in Convenience Stores, which also can be found in grocery stores.
1. Deli FoodsThe deli counter can be a great place to find low-carb foods. Try tuna, chicken, or shrimp salad, that you can either eat on it's own or pre-wrapped (or wrap it yourself) in a low-carb tortilla. Vegetable salads can be good, although make sure the dressing doesn't have a lot of sugar. Guacamole can be purchased to dip cucumber rounds, celery sticks, or other low-carb vegetables in, if these are available. Many delis have roast chicken, which can be paired with a salad. Or make a wrap with lunch meat (check for sugars in ingredients) and a low-carb tortilla.
2. Salad BarsSalad bars can be even better than deli counters in some stores because you can mix and match to your own tastes and dietary preferences. When in doubt about the dressings, blue cheese or plain oil and vinegar dressings are the least likely to have sugar added. Or you can buy your own bottle. How to Find a Healthy Salad Dressing
3. VegetablesSome stores offer prepared vegetables in packages, and some vegetables don't need a lot of preparation. Celery, mushrooms, green beans, Bell peppers, tomatoes, and broccoli require little preparation to be eaten raw, and can be eaten with a deli tuna or chicken salad or low-carb dip.
4. Pre-Made SaladsI'm seeing more and more creativity in pre-made salads these days, way beyond the strips of ham and an anemic tomato slice thrown on top of some iceberg lettuce. I've even seen some that have separate compartments for the various toppings so everything remains fresher. Look for salads with mixed greens, not iceberg lettuce, which is very low in nutrients. Also, look for a real serving of protein, not just a token amount for flavoring.
5. Deviled EggsDeviled eggs can be found in several stores in our area, often near the cheese case or a special shelf for store-made items.
6. Individual CheesesIndividually-wrapped small portion cheeses, usually either wax-wrapped or foil-wrapped, are fairly easy to find and a convenient and portable protein source. Examples are Mini-Gouda and Mini-Bel.
7. Sugar-Free YogurtFinding yogurt that is sugar-free AND contains live yogurt cultures AND doesn't have objectionable additives AND tastes good often requires hunting, but it can be done. Of course, there is always plain yogurt. If that's just too plain for you, try adding berries and sugar substitute. More About Yogurt on a Low-Carb Diet
8. Low-Sugar FruitLow-carb fruits are good choices, if they fit into your diet plan. Check out this list of fruit, with links to carb counts.
9. Sugar-Free Gelatin SnacksFor a quick bite of something sweet, packs of individual sugar-free gelatin cups (such as Jello brand) can do the trick.
10. Low-Carb Meal Replacement BarsIt's hard to find meal replacement bars that are sugar-free and don't have objectionable sugar alcohols such as maltitol. Atkins Advantage Bars are one brand that work for some, although some of the Atkins products contain maltitol, so (as always) read labels carefully.