which wireless window contact/sensors for DIY Home Automation system?

Hi,
my first post here, so I hope this is the correct section...

A friend and I are developing our own HA system. My friend had a new house built and so he was able to have contacts integrated into his doors and windows and wires run to a central place. I've then built a device that reads those contacts and reports the state of each door and window to the HA system.

I, however, live in an old house where I can't just run wires to each door and window. So I'm looking for a wireless / RF solution for the same thing. I found a lot of these sensors but they are all for some sort of commercial stand-alone alarm system.

Does anyone know such a system that can be connected to a computer somehow that would allow me to read the state of each contact individually (not only "all closed")?

Thanks,
Chris