Individual butter knives are a standard inclusion in most flatware collections. These butter knives belong with each of the table place settings. A typical butter knife in a flatware collection will measure approximately 5 to 7 inches in length, and is slightly smaller than a dinner knife. Butter knives are sometimes called butter spreaders. They have a round tip, so that they do not tear the bread as one spreads the butter.
If you are looking for butter knives that belong to a specific manufacturer's collection, RestaurantSupply.com has many different patterns and styles available in case quantities.
Note that a master butter knife is sometimes included with a butter dish set and is meant for serving pats of butter from the central butter dish to individuals, but is never used to spread the butter.