SOLVED, at least for now!e-clips are an equivalent external retaining ring. I needed 2mm eClips.
Yes, can be bought as part of a big kit for US$10 on Amazon.
But I found a very quick solution: our local RC Hobby store stocks the Traxxas #1633 eClip/Cring kit. It contains more than two dozen eClips that appear to work in at least most positions requiring a split-ring retainer. US$2 for the kit.