Looking for interior parts

I brought a 2018 Ford transit 350 HD. The person who owned it before me was cutting up the inside to make it a camper van, and decided not to. It is a passenger model so I would like to find a headliner and side panels for it that he has cut out or left partially installed but cut. Other than the dealership where do I source those parts and what are the side panels where the cupholders are and the walls are actually cold and is headliner actually the correct term for the interior roof panels where the air conditioner and lights are? Thanks