The 30" high is the problem. I was just checking at
www.drsfostersmith.com (in the fish section under ornaments) and their tallest background is 19 1/2" high.......I honestly can't recall seeing a taller one at any of the fish stores I go to either......
Things you could do.....you could buy a standard background and place it near the top of the tank, the remaining 10" at the bottom could be "filled" in with another background of the same type, or plain blue or black, and you could try and hide this with any decor or rocks within the tank.
What I would do though, is to use something else like a plain blue background....2 pieces of that would hardly be noticable. Or you could make your own out of other things. On my 55g I use gift wrapping paper. It's a nice rich blue, fits perfectly and only cost $3. Besides, after awhile you won't be able to see the background because the coralline algae will grow over the back glass (unless of course you keep it clean).
On one of my 20g fresh tanks I am using aluminum foil (this looks surprisingly good). There are many different things you can do for this.....
HTH