Sway bars are harder to find than most other aftermarket parts simply because they are not as common. Addco makes a set and their rear bar is even adjustable! Suspension techniques also makes a set of sway bars. If you can find some, SS sway bars are very nice, if hard to find. If you can get them cheap from a junkyard they're good enough and should be significantly less expensive than new aftermarket units.
While you're changing your sway bars make sure you replace the bushings as the old ones are hardened, cracking, and soft. Even if you're not replacing your sway bars consider changing the bushings, it is a big improvement for very little money. Expect to pay $200 or so EACH for aftermarket sway bars. Nissan sway bars from other year 300ZXs can be had from junkyards for varying prices.