I was just looking at them in my freezer last night thinking they should be used with the delicious yellow fin I bought at New Deal yesterday. I got these at a Brazilian market on Broadway in Somerville - and I apologize for not remembering the name of the market - there are two or three on the few blocks that span from McGrath Highway to Sullivan Square. These are frozen, which I do not mind - because I have them at my behest. Believe it or not (I hope I am correct here) the Spanish or Portugese name for them is plantanos. I will try to drive by later today and see if I can get the store name. Hope this helps a bit.
re: Small Plates
The larger market on Lower Broadway is Tony's Foodland, which is central american, but has a Brazilian butcher. They should certainly have them. Brasil2000 is across the street and is a store and lunch counter. There is another smaller store closer to Vinnys, which I thought was central american.
They are quite common in the freezer section -- Market Basket/Demoulas has them, Shaws is pretty good about stocking them.
In Spanish its probably something along the lines of Hojas de platanos (my spanish is not that great), but the portuguese would be folia de banana. Platanos are plantains, which most frozen banana leaves are (portuguese for plantain is just banana da terra).