Where to buy things in Nicaragua from paper products, printing documents, food items, kid stuff or a plumbing, welding materials and such you go to a Pupleria, Papeleria or Ferreteria these are all small community business that you will find in your community, these would be classified as mom and pop business in the US.
These businesses thrive in the community by adding products and services as needed by their customers, some even will get hard to find products that you might need. Ask the shopkeeper for what you are looking for, if they dont have the product they most likely who who in town has and if not where you might find the product.