I haven't been cel collecting THAT long but maybe 
some of this will help....
The same sites you ordered your cels from, are 
where you should look for backgrounds... they are 
sold in the same way. Unfortunately getting 
backgrounds for your cels is not as easy as 
getting the cels.  Often unless the cel is sold as 
a set WITH the background, you  may NEVER find the 
 background that goes with your cel. There are 
more cels per scene than backgrounds (since one 
background may be the background for dozens if not 
hundreds of cels depending on the scene length).  
Some other collector with a cel from the same 
scene as yours may have "your" background already, 
so you may never have a chance to buy it... either 
that or the background may have never been 
released to be sold in the first place.
Some galleries will sell cels with a lazercopy of 
the background, which is very nice becuase you 
dont have to pay hundreds of dollars for a 
background (backgrounds can SOMETIMES be more 
expensive than cels, depending...)

Good luck!