I got mine from GMpartsdepot.ca. You get both the front and rear liners when you order.
Be aware there IS a difference in the liners for a coupe vs. the convertible. The coupe part number is 84840624 and the convertible is 84840628.
I believe it has to do with the little holes on either side of the trunk liner, they are a bit different in the coupe vs the HTC.
Probably you can interchange them and not notice, but you might as well order the correct part.