I have never seen a reference list for the inner packaging. Not even in the great book of corgi.
The rings can easily made yourself, a toilet roll or clingfilm roll is a good start.
They are made of very sturdy cardboard
Depending on what you buying, Toilet Paper, Paper towels, Wax Paper, Aluminum Foil, Cling wrap, Christmas/Birthday Wrap or garbage bags, There are differnt sizes from 1/2" inside hole to about 3" or even 5" inside hole.