Bus station vendors
You can stock up on fruits, other snacks, water and other drinks for the trip. Prices are inflated, of course, but not too far over the top. If you are lucky they will have baguettes to make sandwiches, too. When I first got to Laos from Thailand, I was so excited to see baguettes with lettuce, tomato, carrot, onion and Happy Cow cheese. I thought I could eat one every day and not get sick of this simple combo. Not so. Within the first four sandwiches I was sick of it.