Use acrylic paint, you should be able to find it at any art
store.  And when I make a fan cel, I do a pencil sketch first
and then put the cel over the sketch and trace the image onto
the cel with an extra fine Sharpie... that's probably the 
easiest, if somewhat inauthentic way to do it.  If you want
to Xerox... sorry I don't know how!  And on some of my older
cels like Maison Ikkoku and Urusei Yatsura, I think the black
lines were actually painted on with a REALLY fine brush!  Oh
yeah, and be sure to use a really fine brush for everything,
it's easier to add detail.  Hope this helps!

Tifa