Cork, and lots of it. This quaint little shop sells it by the sheet, totally natural if that's what…read moreyou're after, and also they offer a variety of cork-based products and effects. I can't think of many shops that sell ONLY cork around here (there are many more in southern Portugal, for example, where cork trees (alcornoques) are ubiquitous.
The shop itself is small-ish and cute, with big plate-glass windows framed in blue. All of the products are on display in the showroom, and surprisingly things are not too expensive. For a project of mine I bought four big sheets of raw cork-board for 20 euros total, or five euros per sheet.
The owner/attendant is a Spanish woman who only speaks Spanish, so if you want to go in there you'll have to know a little in order to help her understand what it is that you want. Pointing works too, I suppose. Cork is "corcho" and board is "plancha", so if you know those two things you can at least get what seems to be their most popular product. This woman is VERY nice and good-humored, and took the time to explain to me how to care for the product and even gave me some advice for my project (picture frames). Without her advice, I would have made some dire mistakes. I'm grateful, so hear me when I say that this place is inexpensive and the service is prompt and robust. Great find in the side-streets of La Latina.