Winchester Model 1200 help needed

I have an older model 1200 12ga that my father recieved as an anneversay present in the early 1960s. I have killed ducks, deer and turkey with it.

I need to find parts for it. Winchester and local gunsmiths will not work on it since it has a serial number that is prior to a certain cut off.

Does anyone know of a reputable gun parts supplier where I may be able to find the parts I need.

The weapon has great sentimental value as my dad taught me to hunt using this gun