Intercom installed, Driveway alarm next

I am so glad that hubs installed a wireless intercom system at home.  Living in four-story house can be a little inconvenient if you don't have it.  We had one before; it was not wireless.  It broke down a couple of months ago and hubs was too busy to replace it.  
We are staying on the fourth floor while my store is on the ground floor.  I had to shout sometimes to call my store helper if I needed to tell her something.  Most of the time I go down the stairs to talk to her.  I know it's a good form of exercise but sometimes it's just too tiring.  The new intercom is such a relief.
He will be working on the driveway alarm next.  Actually our driveway, where the car is parked is kind of safe.  It has an enclosure and a gate that we lock at night.  But hubs doesn't want to take any chances.  We've been hearing about theft and robbery cases lately.  Not in our neighborhood though but you'll never know, right?  He's still looking and comparing prices for wireless driveway alarm.  He hopes to do next weekend because after that, he's going to be busy again.  Do you know where we can buy a cheap driveway alarm?