That is one of the reasons I like to scrap up my background with projects like that, using assorted nuetral squares instead of all the same fabric. That way even if I do run short, it's not a big deal to get more.
Right now I do most of my shopping in my stash, and I rarely have enough of any one fabric with enough yardage for background is there is a lot needed.
Another option I"ll use is to do a gradient look, either from top to bottom, or from the center working out.