Go to your local appliance store. There are many with a track on either side
of the door to accept slide in panels.
max

Looking at pics on the web of many very nice kitchens, they're are lots that
have refrigerators that look built-in - the doors/sides match the cabinets.

I've searched around on the internet and can't figure out where you get
refrigerators that are set up to handle added on panels like this. GE has
some but you have to choose from their panels.

Can anyone educate me about how you'd go about doing this ?

jim