Make your own pen, with a knife and a stick of bamboo!
Step 1: Bamboo
First get a strong piece of bamboo 10 - 20cm long. (This can can be no thicker than 1/2 an inch, and as thin as you like as long as it is hollow.
Step 2: Point the Top
Whittle the end of the bamboo to a fine point.
Step 3: Create the Slit
This can be tricky and dangerous so be careful. Cut the top off the point so there is a 2mm flat bit at the top, then place your knife going from the small flat bit into the centre and push down. (Preferably towards a table or a rock, not your leg). Aim to get the slit 3 or 4 inches.
Step 4: Write
You need ink, I bought mine, good quality, but this works with good or bad ink, it is not hard to make.