Someone probably makes actual threaded caps for those, but I wouldn't know where to start looking.
If you just want to clean up the look of the firewall some, you can check the OD of the fittings and hit the hardware store. Check the plumbing section for some rubber pipe caps that will fit a little snug and slap em on. They won't be too pretty, but they'll keep crap out of the lines and clean it up a little.
You might be able to find threaded caps there too, but again, I don't know the sizes and the AC/heater on my car was long gone before I got it.