I learned to make these from my friend Adrián Marchena while I spent a semester of college in Puntarenas, Costa Rica. I have not personally made these, but I stood next to him in his kitchen as he gave me step-by-step instructions. These are sweet, and almost dessert-like.
You can find plantains in most local supermarkets in the produce section, and they look like bananas, but are more of a vegetable.
They are quite frequently eaten by Costa Ricans, or "ticos" with meals that include gallo pinto (rice and black beans) and some meat product. I LOVE these!