As mentioned, you can use the search option at 
the bottom of the page. 

Since it was fairly recent, maybe you're talking 
about the post user TAVINGTON made about several 
anime series cels/sketches for sale, including 
NightWalker. This person only sells on ebay, so 
you can look up her ebay listings by going to 
ebay and searching under for the seller 
name "Tavington". 

Another person who had NW cel(s) for sale was 
Mina. Her website is http://cels.stage-on.com. 
Her sale page should be linked from there. 

Also, I will be putting a small lot of NW cels on 
ebay, one of these days. I'm just lazy. ^^;;

Oh, before I go, 2 tips:

- When looking up NW cels on ebay or else where, 
remember people like to either join the words 
(NightWalker) or seperate them (Night Walker). 
This makes a world of difference in listing 
results sometimes.

- WATCH THE SERIES! It's short, and sweet ^_^

~Tara