Cool. I've never even seen an adapter like that! I have been slicing with a razor blade on a DC charger cable to split off the positive and negative voltage. Now I feel like an idiot!
If you do slice the cable (use like a box cutter / razor blade), you'll find the wires are quite thin. I usually glob on a bunch of solder on each end, so I have some points where I can hook up alligator clips to each soldered glob end.
I would go on Digikey and look if you can something suitable for your current requirments. They seem to be the source of all this stuff.
It can be a little hairy finding stuff like this on their site, but you just drill through the search results.
Forum post for some links to similiar adapters: https://forum.digikey.com/t/2-1-and-2-5-mm-barrel-plug-adapters/378
Adapter you mentioned (just so you can see what category Digikey puts it in):