Re:
They're readily available online if you do a Google search for a 26.2 seatpost and don't mind a generic one... or are you looking for an original Marin one? They were made by Kalloy back in the day so maybe start your search there?
Dude soak them in white vinegar for 24 hours and the citric acid will remove the rust leaving you a 5 minute job with some polish. Trust me, i did it last week with the original Marin seat clamp from a '91 Pine Mtn and was amazed with the result.