I have a 36" Panasonic CT36D20 television that is 8 years old. For the past two years the viewable area of the screen has been shrinking. The left side is currently in 2" and the right is in 3". Is there any way that this can be repaired and, if so, how?

I assume this is an ordinary CRT-based television set, and not one of the newer plasma or LCD screens.

A "horizontal" sweep voltage deflects the electron beam from the left side of the screen to the right. It sounds like the power supply that provides the DC voltage to the sweep circuit is failing. The maximum voltage is too low. That can be repaired, if you know someone with the proper training and equipment. It may also be possible to simply replace a circuit board to fix it.

If I look for this part will it be a circuit board or a power supply or both?

Could be either or both.

I can order both parts if I know what they are. We have done that before and just replaced both on other appliances and computers. I looked at the parts list for this TV and I don't see any labeled what you called them.