Thanks for the replies! I found a few more: some single serving bags of chips at Rite Aid for around 50 cents. Rite Aid also sells Easter candy in single servings (Russell Stover's, etc.)
I think the best bet is probably bakeries -- buying one pastry/treat there. That doesn't help if I am really craving a prepackaged snack, but a lot of times, treats you buy from a bakery are fresher and taste better anyway!
I have found that Starbucks, Panera, Dunkin Donuts, Barnes & Noble cafes, and Caribou have very good single serving treats. I even went to Dunkin Donuts and bought 1-2 munchkins. It is working for me so far - much better to buy 1-2 munchkins than to buy a whole box of donuts from the grocery store! I wish Entenmann's sold their donuts in single serving packs, though. It seems like the only way I can get those is buy buying a whole box, and I definitely can't just eat one and keep the rest of the box of donuts in my house right now!