A loofa sponge is not a sponge at all.

It's a plant, part of the cucumber family, that you can grow in your garden.  You may be able to obtain seeds from a purchased loofah; just shake it and see if any fall out.  If not, you can purchase loofah seeds at most garden centers.  The seeds are slow to germinate, so start them indoors and transplant them outside later.  The plants enjoy growing on a trellis for support and take about 75 days to produce gourds.  When the skin turns brown, it is time to harvest.  Dry the gourds, then soak them in water until the outer skin disappears, and you've got your exfoliating sponge.  Makes a great gift too!