I've seen them for sale in The Pep Boys, automotive store. Not expensive. Never tried it though, often wondered about it, I would like to install one myself. It didn't look too complicated to me. Our rides have electric door locks for it to be connected to.
I had a Code Alarm system installed. It included remotes which interact with the locks. In addition to direct control it also automatically locks/unlocks when engine is started/stopped and has remote start end quick stop capability. A long range antenna reaches out 1/4 mile too. Had system since 97 no problems. The alarm includes a movement sensor that's useful for a softop.
Yes the Factory alarm was disabled. The Code Alarm is the Sure Start model, probably a different model now. Is your Vette automatic trans? Autos are a lot easier to hook up; with manual you also need a position sensor for the stick. I have one like that for my Jeep, the sensor is made by a different company, from Canada (where else!). Almost all the install is inside the car under the drivers side dash. Agility helps. There are options as to whether you want the starter and/or ignition to be disabled if alarm sounds. I chose NO to that so I wouldn't be stranded if the alarm malfunctioned. We still have the pellet key to slow down hot wiring the car. Of course a professional thief will just drive up with a tow unit and drag the car away alarm blasting and all, we can just run off the idiots who don't plan ahead.
Check around to any decent car radio shops in the area, they should also deal in alarm and keyless entry systems, you might even find one that's running xmas specials right now if you're lucky! I've had one installed in my car for almost two years now and had no problems, although mine isn't as fancy a system as tonylong's. I don't have the motion sensor alarm, but if the car is entered without the remote, then the motor won't start, even with the chipped key...there is a hidden bypass switch I can use if it malfunctions. It's also handy being able to pop the hatch by remote, I say go for it, I'm sure it's an upgrade you won't regret!