Big Pix wrote:
........ there are cases for CF cards, so at a guess, there should be cases for SD cards........
Most, if not all SD cards come with a transparent plastic protective case, makes the whole package a bit bigger and therefore less prone to be lost.
Also, all camera bags nowadays come with dedicated pockets for memory cards, so having them nicely stored in their protective cases and inside the bag's dedicated pockets is probably the best one can do to try to avoid accidental loss.
P.D. I concur with Thomas' aproach of having "all the eggs in the same basket". Better to have one (or two) 4GB SDs than two (or four!!) 2GB SDs.