I'm in the process of installing my alarm system in my house and I'm learning a lot along the way. I have it up and running with future plans in mind.
Are you looking for just a motion sensor? Motion sensors can be inconvenient if you, children, or pets, need to walk around the house. I have a motion sensor but am having second thoughts of putting it in my basement where I originally planned.
I would much rather use door and window sensors instead. That way the alarm is tripped before an intruder even steps foot inside. With a motion sensor the bad guy would have to be already INSIDE the house. That's too late in my opinion.
My alarm isn't monitored at the moment. For the time being it is more to let me know that something is going on and I or my fiancee need to grab the gun.
I've bought all my gear from homesecuritystore.com. It's a legit shop with good tech support if needed.
I'm using a Vista 20P with three door sensors and eight more ready to be installed.
If you're looking for something stand alone they may still have something for you. They have a lot of stuff on their site.