Plantains are roughly 20-30cm long, almost straight & more triangular than bananas.
Green bananas won't work in the tostada recipe, as plantains, especially the green ones that the recipes are very starchy - much more so than green bananas. As they ripen (skin goes black) the flesh mellows & becomes more banana like, but still as some starch.
General rules are the unripe ones need to be cooked twice, like in tostadas & are savory, while the yellow/black ones are fried gently once & are sweet like bananas.
Can't comment on finding them though, as I've only seen them in shops in the US.
It's funny. I see all these comments that fruits and vegs are "cheaper at the farmers market than in the store". Not here. And I've been to Farmers markets where they sell bananas. I have yet to see a banana "locally grown" in the Chicago area.