The most durable and comfortable seats I've made have been built using closed cell foam. The cheapest way I've found to buy it is to bypass the foam shops, and go to a camping store, or a military surplus. Get a camping bed roll. They are approximately 20 inches wide by 70 inches long, at about 3/4's inch thick. That's enough to make about 9 REALLY comfortable cafe/bobber seats. The roll should cost you about $10.
Most foam shops don't want to deal in quantities small enough for our needs. Some may hook you up with a "sample" or scrap for free. I've never had much luck with that though.