Usually you buy the shotgun, then you buy the extened magazine, but not all shotguns can be made to hold more ammo. For example, the mossberg 500 can not be made to hold more ammo because of the way that the barrel screws onto the tube.
Use google, type in something like XXXX magazine extention... you'll have something come up, or call local shops and tell them what model you have/are looking for.