Ramekins are truly ideal for this recipe, as it's meant to be an individual dessert (and wide, shallow ramekins are much better than deeper ones so that the custard cooks through and you maximize the caramelized sugar surface area). However, you can substitute a large, wide pan (ceramic is best), just be sure it can still fit in a water bath inside a larger pan, and, depending on the size, you may need to adjust the baking time.