Dodge/Plymouth Neon seats are a nice replacement. The later ones (99ish and newer) have a lot more padding than the older Neon seats and tons more padding than samurai seats. I have the newer Neon seats in a couple of my samurais. If you remove the Neon seat rails from the seats (have to grind/drill out some rivets and cut a bracket), the front holes are spaced perfectly for the samurai seat rails to fit. You have to make your own holes (I used self tapping bolts), to mount the rails on the rear part of the seat.
I personally wouldn't spend a dime on redoing samurai seats. You can usually get the Neon seats from a junkyard for $20-$30 each.
For the rear seat, you are kinda limited. The samurai seat is so narrow, that very few other vehicles will have seats that will fit (even trackick rear seats are too wide to fit between the samurai fenderwells). I think a jeep yj/cj seat will fit. Or you could have the samurai seat redone....it might cost more than it's worth, but it might be your best option, if you can't find a good replacement samurai seat.