Do you mean D shaped??? If you do, and the material is covering screws in the track, the new material is sold as a roll of flat plastic which fits into the metal trim plate. It is soft when new and is simply squeezed into the D shape as you place into the track.
It is purely decorative, covering up screw heads. The water seal is the butyl tape used behind the metal trim piece. For me, it has proven to be a water trap causing the screws to rust out and thus let water in, in many cases.
It is available almost everywhere RV parts are sold.
HTH