If I understand correctly, you have a better chance of an S-class freighter in Outlaw systems. The capital freighters can only be got from a freighter fight, which happens after at least 5 warps and at least 3 hours gameplay.

You can save before you do the fight and reload if the freighter isn’t S-class. Reports elsewhere suggested it still took hours to find one.

Note you can tell what class a freighter is by looking at a wall in analysis mode in the landing bay.

How do you savescum an S-class freighter now that getting out of your ship creates a “restore point” instead of autosaving?
You have to save before the warp in; you can no longer save on the space station. IIRC you don’t get a save on exiting your ship after the freighter battle, so you can reload and warp back in again.
Don’t do the battle. Go straight to the space station. Get out of your ship, get back in and go out to the freighter. Check its class. If not what you want, reload the bottom (oldest) save. You’ll be back in the spave station. Rinse and repeat until you get what you want.