You probably mean that your house is currently wired with a variety of devices (magnetic reed switches and/or motion and glass-break detectors). The reed switches will be located such that they detect the open or closed state of exterior doors and possibly also windows.
Motion detectors are usually mounted in the upper corner of a room (where 2 walls meet at the ceiling).
All the wiring for those devices will be run to a central location (usually the basement or where-ever your phone line comes into your premises, known as the telco demarc point). At that location, they will be run into a small metal box and connected to an alarm panel. Naturally there is a variety of makers of these panels, and they will also have a variety of matching control panes (usually just a simple numeric keypad with some LED's and a beeper) that are wired to the alarm panel and located at some convenient location (like near a main entrance / exit point).
It's not clear if you are missing both the alarm panel and control panel, or just the control panel. If you are missing just the control panel or keypad, then you will probably have to check on who makes your alarm panel and then buy a matching control keypad panel. Look in your local telephone directory under security alarms because that's likely going to be your only local source for another keypad. There are some large security and alarm products companies on the internet that will probably be your least expensive option.
If both your alarm panel and the control keypad panel have been removed, then naturally you'll have to replace both of them, and you can choose any manufacturer. I'm familiar with DSC panels, but there are others.
Also note: It's typical for the alarm panels to have "installer" or "master" codes that are known only to the original installer of the system and will not be known to the homeowner. You may have difficulty in programming and using the system unless you have those codes, along with the installer's manual for the system (which will be more detailed than the user's instruction booklet).
I've installed my own security systems at several locations so I know something about them, how to install them, program them, wire them, etc.