That has to one of the hardest aspects of this 
hobby.  Sure there are thousands upon thousands 
of cels used to make a series/movie, and many of 
them are very nice. But there are a select few, 
from certain scenes that it seems that everyone 
and their cousin wants. Then those few who have 
been "blessed" enough to obtain that cel from one 
of those ever coveted scene, "lock" them away 
into their gallery, never to be sold. I've been 
told that too many times it seems.  

All I can say is that there is some hope into 
obtaining those particular cels you want.  I'm 
sure you have heard this said over and over 
again, that if you are patient long enough, you 
will find another cel.  Yes, I know that, that 
saying is not always true, but sometimes it does 
come true.  For example, after about 18 months of 
searching for a particular scene from Shamanic 
Princess, and finding two cels from the scene in 
other people collections; the exact cel I had 
been looking for, was put up for sale. Cost me a 
small fortune, but some things you just need to 
splurge on.  Another one of my "great" patience 
is rewarded stories. I've been looking for a cel 
from Lodoss OVA, where Deedlit is splashing water 
on her face, since I saw the genga of it for sale 
on Taros site over a year ago. I've seen one cel 
from that scene, and I missed it.  But another 
cel appeared the other night, and I have the 
confirmation now that it is mine. ^_^

Just keep on looking and waiting, I'm sure you 
will find another cel from the same scene or 
another that you want, that will make it all 
worth it.  

Fenris