Anime Time generally has anime cel backgrounds for 
sale. Go to this URL to see what they currently 
have:

http://www.rinku.zaq.ne.jp/bkacr201/back%20ground.
html

The two that are currently there are from Beast 
Wars, but they have had other series backgrounds 
in the past. Maybe they could find you something 
nice if you sent them an email, detailing what 
you're looking for. Just a thought.