The only red back shell I have found for my 820 was a red wireless charging shell. I ordered it on November 29 and got a email that it was shipped on December 4 but would not arrive to me until December 30. Ordered thru Amazon but I can only guest that it is being shipped from someplace over seas (I live in USA) and thats the reason for the long shipping time. Cost was 16$, and I ordered it because I could not find another other red back covers besides this one. Don't know why exactly but I have the feeling I will never see it.
I do have a incibio feather light blue cover (goes over the back cover) that is nice, very thin abut 1mm. But it does add a little bulk to the 820. But it is available in red if you can not find a red back cover. case.