I have a ding in my rear passenger side bumper. It appears that you can replace just the passsenger side section since the bumper comes in three pieces. Does anyone know where I could get a used one? Is the limited runner bumper compatible? Can anyone provide the OEM number for the SR5?

Your local wrecking yard, or have them get on the hotline and locate one for you.
eBay, SERIOUSLY, just keep looking, watch the shipping though, there are "parts cars" on there all the time. I bought a perfectly good radiator for $9.00 cost me $19 to ship.