If you can't find a used set of Shoot Ogawa's Ninja Rings (the original sets, NOT the new crap
that's for sale everywhere and are not really endorsed by Shoot, despite what the package says. I
know - I asked him about it at a lecture), then your next best option are Matthew Garrett's rings.
Smooth welds and excellent finish. A bit thicker than I'd like and with a larger gap in the key than
is necessary, but otherwise, a fine set.