Those permanent ink pens from Itoya or Y&C 
seems to work pretty well on cels.  I tried it on
some cels several years ago, the lines still look
great and no fade out.  Personally, I like to use
the Itoya ones better, it's called Itoya: 
Finepoint System, they have different sizes, 
0.5 and 0.3 are great on drawing lines for cels.