Hi there, welcome to collecting :)

First off, a great resource in general is 
Cels.org (a clickable link should be to your 
left).  You can find most of the "usual" dealers 
listed there and there is a searchable anime cels 
directory as well as other information.

However, I know I've seen Noir cels at 
AnimeChaos.com, Anime-Link.com and Asylum-
Anime.com at least, as well as some at 
NichibeiAnime.com (though that may have been at a 
convention, you can email them and ask if you 
don't see any on their site).  I'm sure I'm 
forgetting a few.  ^^;;

Additionally, there are Noir cels on Yahoo Japan 
(where I got many of mine).  You can search Yahoo 
Japan in English using Rinkya.com.

Keys