"Tichy Train Group" makes separate windows and doors. e.g. https://www.tichytraingroup.com/Shop.aspx?SearchValue=8036 or https://www.tichytraingroup.com/Shop/tabid/91/c/ho_framed-windows/Default.aspx. I bought a "life time supply" many years ago in the US to balance shipping cost. May be MB Klein was the online shop I used.