If you want to go seriously retro, then what you want is a saddle with "hair pin" springs:
http://www.ebay.com/itm/LARGE-HAIR-P...-/190488112738

Not exactly cheap though, I'd agree.

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If you want a leather saddle because of the way that they look, there's nothing wrong with that. I don't much see any technical advantage however; at the very least certainly the wet-weather durability is lacking.

Leather saddles predate pneumatic bicycle tires, and they have generated discomfort complaints for about as long as they have existed. That is mostly the riding position necessitated by the upright bicycle however.

Lots of people like how leather looks so they still make chairs and seats out of the stuff,,,,, but no modern vehicle builder of any other type claims that leather is a superior seat material based only on technical reasons.