You're going to be looking a long time. Few were built in that configuration. They are out there, but I wouldn't be surprised if you have to travel hundreds or thousands of miles to pick it up, and it will probably be a months long search across all states.
I did see a 1996 GMC C1500 Sierra SLT a few weeks ago with the 5.7/5-speed manual combo. Factory leather buckets in blue of all colors. White exterior, extended cab. All original, that's how GM built it, probably for a special order (owner was not the original owner). Not for sale. The owner knew what he had. It was the first one I have seen like that in person, might be the last one.
The vast majority of the manuals built have the 4.3, but of those, most do have manual windows and locks and will be W/T package trucks. The problem is, W/Ts before '99 did not get the split bench. There are plenty of seat swap options for these trucks though.
Unless you are okay with this being a very long, challenging search, you may have to make the truck you want. The good thing is, these are some of the easiest vehicles to swap parts to your liking.
You will likely need to start with a 4.3/manual/W/T for your base truck because that's all you are likely to find somewhat readily available, and even those are rare. You can swap in a 5.7 and use the 4.3 wiring harness and PCM, BUT you will need to add wiring for two injectors on a multiport Vortec truck and have the PCM reprogrammed. I am not sure how much more complicated this would be with a '99-'02 GMT800 going from 4.3 to 5.3, as those were different engine families with the 5.3 having individual coils.
See here:
http://lt1swap.com/v6_to_v8_vortec_conversion.htm
A TBI truck ('88-'95) will be a little easier.
Split seats or buckets from another truck or Tahoe/Suburban will be bolt in.
There's no easy way to get the truck you want. You're not going to find it quickly looking on Craigslist or hitting up used car lots, but it's not impossible and you can always make it the truck you want.
Good luck.