I've bought cheap cels in the past for the 
backgrounds, just like you're explaining now.  
Then, if I ever do part with the cheap cel, I just 
make a copy of the BG to keep with it.  ^_^  That 
way, you get a nice BG for one of your cels, 
and you don't have to feel bad because 
you're really not seperating the cel from the BG!  
^_^