Oh dear.... someone reminded me of Pololu
New since I last looked...
... an infrared heat sensor, $20
SOMEONE has to make an electronic pit viper!!
If one of these sensors, with suitable optics (not hard) was mounted on a servo controlled rotatong-in-horizontal-plane surface, you could make a "toy" that would seek left-right-left-right... until it saw something hot... i.e. you! And then it would track you.