I haven't ever seen those for sale, back to '75 when I was first buying framebuilding parts. I'm thinking you could make them out of some of the cable stops that Ceeway sells. Or do a little blacksmithing.
I was going to suggest you go around with a hacksaw on bulk trash day, but most of those bikes have something you'll have to blacksmith into shape anyway. My LBS that specializes in used bikes chucks large numbers of frames. I've never asked to look at the pile that's going to the recyclers, maybe I'll do that this week.
My thought with the Ceeway cable stops is to take the bottom pedestal off of it. I have some like those that would look a lot like the one you posted if the pedestal was gone. Seems like "ART 152" would yield something useful after filing. Looks like you could cut the domed end off of "ART 453" too.
If I was making it I would chuck a short section of rod into a drill and clamp the drill in a vise and run it at medium speed while shaping with a file or a grinder running in the counted direction. when I had the shape I wanted at the end I would split the ball with a cut-off or saw, and drill the holes for the cables, then saw off the rod. I have a bunch of lathes so I would actually machine it, but I used to make quite a few parts on a drill before I got the lathe. Some people use drill presses to do work like that, just be sure to respect that even a 1/3rd hp motor is not something to mess with. So lets say you are cutitng off by holding a hacksaw against a spining part. No problem, but stop before it cuts so deep there is any risk of a jam. That saw starts to whip around with you holding on, and it can hurt,, but it might help you to get a shallow groove then do the rest by hand.
Yeah, I've been turning that sort of thing round in my head for a while. I also found several Ebay framesets -- mostly French bike-boom crap -- that have them. What with the fixed gear craze, I could probably buy one for fifty bucks, cut off all the braze-ons, and sell it for $175.