Nadesico was not produced by Gainax (as far as I 
know).  It was produced by Xebec, which is 
affiliated with Production I.G. (which is also 
producing the latest Gainax OVA, Furi Kuri).  But 
you are correct in stating that Nadesico cels 
(like Evangelion cels) were never officially 
released (and thus making the ones that have 
snuck into the market highly rare and expensive).

You've got expensive tastes, my man.

It appears you like Sci-Fi and probably mecha to 
boot.  With that in mind, the only shows you will 
find with still affordable cel prices are 
Gasaraki (great show), Gundam (outside Gundam 
Wing & original Gundam), and Brain Powerd 
(brr...).

Dual! Parallel Trouble Adventure may the closest 
thing to Evangelion as you can get in terms of 
mecha, but cels are few from that series because 
most of it are digitized.  And thus the cels from 
it are not cheap.