Retainers here: Amazon.com: SockLocks: Home & Kitchen
As for the plastic bit, the ones I've made started out as 1"-3/4" PVC reducers.
If you use a dremel to carve a recessed trench inside the lip of the coupler the natural springiness of the sock ring (or CCI ring) will keep it from popping out of place.
The blue one I made first, many years ago, and overestimated how deep I needed to dig the trench, which is why it looks damaged. image sharing