Can I use betel leaves (paan) as a replacement for banana leaves (e.g. to steam fish)?

I agree with Cinnamin, betel leaves are most likely too small to steam things in (unless what you're steaming is paan-sized. Then they're perfect). But if your recipe calls for banana leaves, the fish is probably much bigger than paan-sized, right? If you really want to use betel leaves, you could stitch them together with twine. Stitching leaves is actually pretty common in India.

To get betel leaves, you must be pretty close to an Indian store. They might have banana leaves. If they don't, ask if they can get some for you. They might! There's also other leaves you can try: colocasia leaves (from taro plants. These are used in making patra, or patrode, and are delicious); young lotus pads (or water lily pads); leaves from beets; even chard. Each of these will, of course, make your food taste a bit different than a banana leaf would have.

You could also dehusk a corn cob and use the husk. Or you could buy dry husks, like with tamales.

I use foil or parchment paper when I don't have banana leaves and want to make Kerala masala steamed fish. Betel leaves are rather small

Banana leaf tends to give subtle flavor and different charred smell when you use it for cooking. I never used betel leaf for cooking, but remember that it is for chewing tobacco. If I don't have banana leaf on hand, I'd used aluminum foil or parchment paper to sub it. Slightly different, but you don't lose much.

Isn't paan some sort of mixture with tobacco and areca nuts? Aren't the leaves a bit small for this sort of thing? Where the devil do you get them, anyway?