https://www.lwdparts.com/product/tra...5pr-field-gun/
I needed a pair of lunnettes for the two guns I picked up, and bought them from the link above. There was a difference in the style of nuts, but the rings themselves were identical to the Canadian version.
If your ring is worn, you can also take the old assembly to a trailer place who can cut the ring off and weld a replacement ring onto the bracket.