It really is as simple as asking the BG owner for 
a copy, but finding said owner would be difficult.

I've had to make a few copies for people that 
ask.  After trying out a few places, I would 
recommend using Kinko's self-operated color 
copiers instead of letting them handle it 
themselves.  The color adjustments are pretty 
easy to do, and you won't have to worry about 
them touching the BG.  In fact, I found out that 
I did a better color match on my own than the 
people at the counter, sheesh.  However, for dark 
BGs, you may never get a decent copy.

Now, finding the owner of the BG would be very 
difficult to do.  Browsing through a gazillion 
galleries would be burdensome, but you may be 
lucky. . .or you could start a 'help me get a 
copy bg for this cel' thread or something.

You could also try to generate a BG from DVD 
caps, but the resolution wouldn't be high enough 
to make a decent background.