Pan & Tilt Sensors
There are a few pan and tilt kits out there. I like the one from Lynxmotion - it is made of black anodized aluminium and it is a good compromise between size, weight, and durability.
You can either buy the complete Lynxmotion Pan and Tilt Kit (With Servos) for around $36 or you can buy the Lynxmotion Pan and Tilt Kit (No Servos) for around $10 and 2 × HS-422 Servo Motor, which can be cheaper sometimes.
The original Assembly Guide is pretty easy to understand, but I modified it a little. The pan servo can be used with the full 180°, but the tilt servo is limited. So, I modified my tilt servo by removing the disc and turning it 45°. I still cannot use the full range, but it gives me some room for error and it is less likely that one bracket jams into the other. The disadvantage is that it changes the midpoint and the servo needs to be calibrated.
I did not like the "push-in fasteners" and I used M4-.70×10 hex cap screws, M4-.70 hex nuts, 4mm flat washers, and 4mm split washers instead. Also, the constant motion can break the wires or moving cables can get in the way. I added some Sugru to the wires as a strain relief and to keep them from moving.
Ultrasonic Distance Sensor
The first sensor I tried is a ultrasonic distance sensor. I like the Ping sensor from Parallax.