One of my favourite shops of Santiago is the stationary shop Mayso. It is in Avenida Rosalia de Castro. They sell online but have more variety of products in shop. And you can pay by card.
They have a wide variety of products. But above all in the products related to the plastic arts, because you can find very specific products that is more complicated to get in other stationery shops.
The last time I was there it was in November, because I had to buy paintings for a biology project. Maybe I will go back soon, because I need to buy materials for other projects.
