I found a bunch of PIR sensors on eBay. They are mounted on a pcb that was produced for a hands free set for mobile phones. I like to describe here how to prepare the sensor for the usage in robotics projects.
If you don't know what a PIR sensor is, just have a look at Wikipedia http://en.wikipedia.org/wiki/Passive_infrared_sensor
. The product where the boards comes from can be bought here http://www.greasemonkeyconversions.com/10609/Com_N_Sense_Hands-Free_Kit_(Nokia_3310_etc).shtml
. I bought the boards from a seller named "kalleb" on eBay. A search of the seller or for the subject "PIR INFRARED SENSOR" leads to the offer. He still offers some boards.
On the boards you can also find some switching voltage converters. I used them in an other project http://www.instructables.com/id/SLVOL8FFBGW8AF4/
where I needed to get +-15V out of a +5V supply. There are other useful components as well, but here we only need the pir sensor and the op amp that prepare the pir signal for direct usage with a microprocessor.