Most copy places now use laser color copy 
machines.  It wasn't always so, and there would 
be a big diff between a color copy made with a 
laser unit and a regular color copy machine.  
Color density and saturation was much better with 
the laser machines.  

Nowadays, I recommend Kinkos, because it's self-
serve, and you can trim the 11x17 paper using 
their awesome cutter!

Since usually only one cel from any given scene 
ends up with the original background, I've made 
copies of mine whenever I have more than one cel 
from a scene.  I don't think studios would 
bother.  Some Japanese sellers will list their 
cels as having a color background, but again, I 
think they would have made it, as I've noticed 
that with dealers who sell multiple cels from a 
particular scene over time.