I have read here somewhere that the sundowner are obsolete.
The P&A catalog is still showing the Sundowner Seat, part number 53432-02 as a valid part number, still available. Here's the catch though: You MUST have either part number 51120-01, or 51098-06. Both of those part numbers are for the fender base plate that is REQUIRED to fit the seat on the bike (because of the passenger pillion built onto the Sundowner part number listed above). Part number 51120-01A will NOT work with the ''06 SEVROD...the rear fender is too wide. Barnett Harley Davidson is showing part number 51098-06 to be in stock on their web site, but you'd have to call to be certain. Both base plates are obsolete part numbers, but you might find one that some dealer has laying around in his inventory, but you're going to have to search. GOOGLE either of those two part numbers and see what you come up with.
There was also a Tall Boy seat offered back then...part number 52927-05, but it also requires the same base plate.
Your other option is a Corbin, which requires no base plate. They offer a Hollywood Solo, A Dual Tour, and a Young Gun. Corbin may also be able to tweak the seat a little bit to move you back and inch or so. Go here to view:
http://www.corbin.com/harley/vrsca/cvomenu.shtml If it were me, I'd go the Corbin way, unless you luck up on a base plate. Corbin seats are very firm, but are high quality and more comfortable than they may seem....firm is good.