Where can I get replacements for those? I am talking about the plastic that is used to cover the little metal strips that you fold over to hold your harness down. They are found along the rear trunk and other places in the car. Perhaps there is an equivalent that can be found at a hardware store?

Heat shrink tubing.

Agree w/Jeff B - heat shrink tubing is perfect.

Doubt that that's what is was originally, but you can't tell the diff today.