I tripped over a site that had some really expensive 3-spring seats, and can't seem to find it again. The price seemed crazy to me, up to $300. However, there are some more affordable ones at Custom Motored Bicycles
The "Leather 3-spring saddle" at the top looks good, there's another 3-spring farther down, the "Crash Rail Seat".
They look like they'd smooth out the ride very nicely.