The type we most often use here in the office is a "push button" combo lock box. These can be purchased at either Orchard Supply Hardware (OSH), Home Depot or Lowe's. If you cannot find one there, you can also purchase a combo lock box at any fine locksmith store. Programming the lock is very easy to do. If you're using the push-button style, there box contains a small plastic screwdriver that you'll use to move the pins from up to sideways or vice-versa. There are also combo boxes with rolling letters that you can use, but they're not nearly as secure.
If you're looking for a lockbox such as we have as real estate agents, however, you'll need to be an agent to purchase and use the box because these are tied to a computerized source for access and information. If you would like to have a computerized or professional lock box that can be accessed by realtors, you'll need to speak to another real estate agent to obtain one.
Good luck! Try the push-button lock box. They're affordable and very easy to use!