Cels Forum

Either you go to Studio Gainax and steal them yourself,
or you get a friend who has a friend in Japan, who either knows a friend who works for Gainax who can steal them for you. If you find one, expect to pay 450$-1500$ for one. Unless the person doesnt know how much it is worth, which I highly doubt anybody doesnt. (and owns one.)

Dont want to take that route? then you are best off either buying the Cellu-club reproduction cels made by Sega, under licence of Gainax. they are nice, cheap, but they are not production cels.
You can find some at anime-taro,
http://www.anime-taro.com

Or, you can wait until Gainax releases them officially, which nobody knows when, it could be years, decades from now.

Now I cant wait until mine comes. (I took the first route.)

Later!

Kur0ryu