Sounds like the milling setup on a Walker Turner. I don't think you'll find what you're looking for. Both ends of an adapter like that would be too big for the threaded collar to fit over. I made a replica of the original for mine. It's just a milling cutter holder with a shoulder for the collar to bear on, and the JT33 taper on one end. Mill holder is just a 3/8" or 1/2" hole with a setscrew.
In 1963, I got a nearly new drill press at an estate auction. It had the 1/2" Jacobs chuck with locking collar. Back in 1963, I was able to then buy the other JT33 locking collar accessories: collet chuck for router bits, Drum sander and grinding/buffing wheel arbor. I think drill presses with that spindle went out of production about fifty years ago, so there would not be a great market for ER collet chucks to fit. There might be some of the old accessories on eBay occasionally, but I think the Jacobs chuck is the only thing still made. And that will be Chinese. Back in the olden times, Jacobs chuck parts could be bought separately, but maybe not now. As I recall, the chuck locking collar is retained on the chuck body with a snap ring.
The locking collar's main purpose is to make removing the chuck or other accessories relatively easy, as opposed to hammering on Jacobs wedges. Most drill presses have a plain JT33 without the male thread and the chuck, once installed, is almost impossible to remove.
About 1976 I bought a Sears Craftsman drill press, made in USA. Turns out it was made by Emerson and was also sold under their brand name. It has that Jacobs chuck with the locking collar. As I recall the Sears Craftsman tool catalogue at that time showed the collet chuck for router bits and other accessories that Larry describes.
It was the beginning of my home shop that now includes a Bridgeport and a 10EE.
A hobby that got out of hand...