When I was schooling for my BA in Animation in 
Los Angeles, all us students purchased from a 
local speciality store that specialized in 
animation art products.  They sell authentic, 
punched cel acetate and authentic cel vinyl 
paints.  I always buy the authentic materials for 
the actual paint and cel, but I use simply 
Rapidograph pens for inking.  It dries pretty 
quickly and is easy to remove mistakes if you 
catch it soon enough after making them.

Cartoon Color has an online website for ordering 
and an entire catalogue of pre-mixed colors and 
you can even order custom cel-vinyl colors:

http://www.cartooncolor.com/

Hope this helps.