You can look at www.overtons.com
, they have generic ones that might fit. Also, contact your manufacturer to see if they will make one for you. I am about to have Sanger build one V215.
If you're gonna have one made and are looking for design idea, Supra makes an AWESOME jump seat for their Launch 21v (http://www.supraboats.com/launch21v.html
). It lays flat when not in use (although your walk way goes away) and can be raised to create a seat facing the rear AND an extra seat in the playpen. Very nice!