Traditional bain-marie pans are used to keep sauces warm during a restaurant service. They have a tall, cylindrical shape with a handle near the top, and are designed to be set in a pan of hot water.
Some version of a bain-marie is essential for sauces that are made in advance. Unfortunately, tinned-copper bain-marie pans are expensive, and aluminum reacts with too many sauces. Stainless-steel bain-marie pans are difficult to find.
Good substitutes for bain-marie pans are the cylindrical stainless-steel inserts used in steam tables. When new they tend to be expensive, but they can often be found in good condition secondhand. They can also be filled with hot water and used to hold whisks, tasting spoons, and ladles during the restaurant service.