[Hi Volt]

You are looking for the right hand box side. The damage is the rear quarter of that panel but it is one piece front to back. Commonly these are tack welded to the bed floor and inner box liner. You might get lucky and find a complete box in good or easily repairable condition at a wrecker or a truck mod shop that has converted one of these trucks to a flatdeck or service truck type rear box ( I have seen a few modified trucks like that around ). Might be cheaper in the long run instead considering the cost of the new side, labour to re+re, let alone paint.

This is also why we buy insurance, what about comprehensive coverage on your insurance ?, even whatever your deductable is should be far cheaper than going it yourself. Technically collision might also apply as you did somewhat collide with something... Maybe ask your insurance rep.