I appreciate all the help people offered here. I'll post a follow-up just in case someone searches for info on this topic in the future.

For various reasons, I've decided to install bar-end shifters instead of STI. They're on order and should be in my mailbox in a week or so. Obviously, I still need the housing stops.

The shifters do indeed come with cables, housing stops, everything needed for proper installation.

And it turns out my bike has standard shifter bosses/studs brazed onto the downtube. Those funky, splined "plate holder" pieces shown in the picture above were simply pressed on over the studs. A little careful prying with a screwdriver easily freed them.

Now I'm super excited to get the bike all set up how I want. It's a good bike now, and will be even better with the new levers.