How or where can I kitbash or find windows for the passenger side and doors for passenger cars.
Sheet Styrene. Clear for the windows, opaque for the doors. It cuts with an Xacto knife, or sissors and glues with styrene cement. Interior doors ought to have glass windows in them. Hobby shops sell small (6*8 inch) pieces of styrene. You might be able to scrounge some scraps off your local glass and plastic shop.
I also use the clear plastic from boxes, the kind build-ups come in.