Three pages of them for sale here at:
I'm certain Brent could sell a lot of seats if he wanted, the store here at paddling net mostly carries hard to find accessories and ads for other suppliers.
Most of the sit on top seats do work well in either the back or the front position, but you need to adjust the straps to move the seat to a different position or a different boat.
Cartop Kayak Carriers
YakCatcher Rod Holder