If you're not that concerned that the background 
won't be the EXACT match, but think something 
would look nice behind your cel, you have a few 
options:

1. Some folks have taken to getting some 
watercolors and nice paper and painting their own 
backgrounds.  Mostly, this has been used to do a 
color wash behind it (giving the impression that 
the characters in your cels hang around at 
sunset), but it can work rather nicely.
2. At cel dealers in Japan, such as Mandarake or 
AWS, you can get bags of loose backgrounds.  These 
can be used for nice effect or just to do 
something silly (I almost bought one just to put 
an Armored Trooper Votoms cel against a pastel, 
shoujo-kiddy bg).
3. You can also invest in nice colored paper (be 
sure to check if it's archival-safe or not) and 
just use that.  An art supply or framing shop 
should be able to offer you a range of nice papers 
for a striking effect.