Skeleton keys are not the common household item they once were. Most homeowners today use feather keys for their locks to operate the pin and tumbler-type mechanisms that are primarily in use. Pin and tumbler locks are more secure than locks used in conjunction with skeleton or antique keys. A locksmith can replicate a skeleton key to fit your locks if they have a key that is similar to the one you have lost, misplaced, etc. to fashion a new one from; however, because of security concerns, locksmiths will recommend that you instead change entirely over to a modern system or that you, at the very least, supplement the antiquated locks with a more secure mechanism. Also, it is not a 100% guarantee that they can procure a key similar enough to the one you are trying to replace as not all antique keys are still in production today. To find out requires you to bring the lock itself to the locksmith or to have the locksmith come to your house.
Bringing the lock to them is the preferred method as they have more tools available to them in their shop. It is also the cheaper option because the billable hours would not start until you bring them the lock to work on as opposed to them charging you a premium for travel to your house beginning when they leave their shop.
Bob at Specialty Lock and Door Co. of Waynesville, North Carolina estimated the cost of replacing a skeleton key at $50. The cost of the key (if he happened to have the correct one on hand) is about $25, the time involved to make the key would be about an hour, he charges $25 per hour for labor resulting in a $50 cost to you. Bob recommends changing all antique locks to modern deadbolt systems rather than potentially wasting your time and money. Yard sales and estate sales are two places to search for keys on the cheap. Also, the cyber equivalent to yard sales and estate sales are sites such as eBay and Amazon which offer countless options to replace your keys for less than $10.