Folding Bike Seat Post Replacement
You want a separate saddle clamp, that clamps round the top of the post and the rails of a standard saddle, something like this.

See also http://sheldonbrown.com/saddles.html#seatpost

http://sheldonbrown.com/harris/seatposts.html (scroll down to 'Traditional "pipe type" seatposts, use with separate saddle clamp'.)