Banana leaves are very common item in Thai cooking. They can be bought frozen from Asian supermarkets and in some places fresh. A common misconception is that they have a banana smell, they do not - they smell like leaves! The reason we use them is not for the smell, but because they are waxy, so food is easy to remove from the leaves. They are also natural and organic, so no waste and also they look good for presenting food.
If you buy them fresh they will go brown within a few days, so freeze them or keep them in the fridge. If a recipe is made in banana leaves, you can use oiled tin foil instead. Remember they're a cooking implement not an ingredient. I really should have out these in the 'Equipment' category.