An IR Pen is relatively easy and inexpensive to build.
It is comprised of an IR LED, switch, battery and a pen housing. 

The IR LED positive terminal is the longer/flat wire.
It is connected to one terminal of the switch and the other to the positive wire of the battery.
The other terminal of the LED connects to the negative terminal of the battery holder.
A resistor isn't needed since we are using a 1.5V power supply.