This may not have been brought up by as far as I know only Realtor agents can have access to the Realtor lock boxes. The reason is because the whole goal of the real estate lockboxes is to give exclusive access to the agents and to assure the complete security.
If you are looking for a consumer product, RYOBI makes an electronic lock box that has up to 10 unique programmable access codes. It can be purchased at Home Depot for about $40. Not the most user friendly, but it works.
Henrietta: The supra key is available to licensed real estate agents, so if you wish to have a key box on your house you could buy key boxes from a retail hardware store....but why sell yourself......hire a professional agent who has knowledge of your area and sit back and relax.