Brand new blacked out leather Corbin seat fits GTS 250/300. Bought it, it arrived this past week, rode around the block with it, and decided I liked stock seat better. $300- free continental US shipping.

Link to Corbin site:
https://www.corbin.com/vespa/vtgs2506.shtml
Seat Full View
Seat Full View
Detail
Detail
Back Detail
Back Detail
Bottom
Bottom