I found it incredibly easy to buy organic products in Sevilla, where we lived three years. As a smaller city, there aren't too many big supermarkets (either mainstream or natural) like we're accustomed to in the US, but smaller shops were dotted throughout the downtown city center. There was also one coop that apparently has been around for 30 something years or more. I also had the privilege of watching what was a little corner eco-tienda when I first moved there triple its size as the stores next to it vacated and the owner claimed that space. Also during our three years living in the old city center, we saw at least five juice bars and natural foods style restaurants open up (serving your mix of salad varietals, smoothies, acai bowls, that kind of thing.)
Taken altogether I interpreted that growth to indicate that when we'd arrived in Sevilla, the notion of organic and health foods was still a niche market, a bit fringe, but that it was becoming more mainstream. Probably not coincidentally, I also saw any number of new yoga studios open during this time, further confirming that the consciousness of these ideas, activities as well as ways of being/eating in the world was somewhat nascent in that community on my arrival but was gaining steam.
I also found the mainstream supermarkets all carried any number of organic products, and as a gluten free eater for many years, I was relieved to find it very easy to maintain my diet while living in Spain (both for products purchased for home as well as eating out). Finally, I appreciate the GMO products are labeled in Spain (a mandate of EU law, I believe). In the US, the corporations that make those products have such power in the government to keep that information hidden from consumers.
Lastly, having met a few people in the agricultural industry out of Huelva, where much of the produce sold in local markets in Sevilla comes from, I was told that while not strictly organic, the proximity of that national park there, bans use of many of the more harmful, chemical pesticides in that region. Therefore, I felt comfortable buying berries and other products from there, even if not 100% organic. I like purchasing food regionally and locally as much as possible.
So, my take on the organic and natural life in Spain, that it's readily available certainly in the larger cities and probably the areas in close enough proximity to them. Smaller villages in Spain? Hard to know. But again, my experience tells me that some portion of products available there would be locally harvested or reared, and that with a little digging, it probably wouldn't be too difficult to get some insight into the production practices involved.