I think you have sort of a wrong idea of what a cel background 
is - they're not "put in" or painted directly on the cel, if 
that's what you mean. They're basically acrylic/watercolor 
paintings (correct me if I'm wrong) on a thick watercolor 
paper, although in some cases the studio will make laser 
copies of a background for a cel. 

As such, the cels are usually attached (by staples or tape) 
to the background when you buy them, and the background is 
usually larger than the cel(s). I've seen a couple of sites 
sell backgrounds, but they were all sold out when I visited. 
You can easily make your own background using the paints 
mentioned above and using acid-free watercolor (or other) 
paper. In my case, I just add simple backgrounds in Photoshop 
and therefore my cels are kept in their books while I can 
enjoy them on my screen (I could even print them out, but 
nah)... If you do make your own bg's or buy them, remember to 
separate the cel from the bg with either a sheet of cel bag 
plastic or with a cel bag.