For a boat made 18-years ago and had multiple seating options, I would not expect to find any OEM seats still collecting dust on a dealer’s shelf. A seat like that would not be a part a dealer kept in inventory.
Boston Whaler doesn’t hang on to optional components for boats very long. They often sell them off to local Florida shops that specialize in reselling the odd OEM parts left over at the many boat builders down there.
But even at one of those places to find an OEM seat for an 18-year-old boat would be unlikely.
I suggest you post a notice of wanted to buy in MARKETPLACE and hope there is another 18 DAUNTLESS owner that bought the optional seat and backrest, but no longer uses them and wants to sell them.
For a recommendation for a supplier or vendor of upholstered seating for Boston Whaler boats see the REFERENCE section and the article on recommended vendors.
Here is a hyperlink:
OEM PART SOURCES: Recommended Vendors
http://continuouswave.com/whaler/reference/OEM.html