For a boil kettle, I would go with a stainless 1/2 inch ball valve. Of course, you can get a butterfly valve if you really want to. Either one is going to work fine.
You can get a good price on a 2 or 3 piece SS ball valve at bargainfittings.com. You might also want to check out weldlessfittings.com. Be sure to get a hose barb as well.