If you are talking about the Pirelli seals attached to both the bottom half and top half of the camper, you can get it from Alaskan factory, I would not suggest you use anything but that product as it does last and is perfect for this application.
They may also be able to help with the actual window pane gaskets, but the window frames will need some butyl sealant to waterproof them and prevent seepage into the internal wooden framing which leads to rot.