I've been buying up the 40 disc cases on Amazon : LINK
They tend to fluctuate in price, but £6-7 is average each.
They are great for saving shelf space. I found you can get all 6 series of Lost in 1 case! or Futurama, Life on Mars & Ashes to Ashes in 1, plus I have a few called things like War Movies or Action movies.
A huge shelf saver!