Last I spoke with IceFalcon (a few months ago?) he doesn't have a source for individual solid rings (9mm stainless is what I asked for, though). Rivited mail isn't really any "harder" than butted, but it's easily twice as time consuming since you have to weave the rings slightly different to keep them from deforming, and having to constantly swap needlenose for riviting pliers and then setting the rivits. When I finished with my mail skirt a week or two ago, I had 3 blisters on my hand from the riviting pliers.
I found an interesting post on MyArmoury about the rivited mail. Seems that some places have someone who just builds "squares" of mail, and then to make chausses, hauberks, haubergeons, etc, they assemble the squares into the final shape. That explains the always tube shape, and possibly why some rivits are set poorly - multiple makers working on a single item.
I've got the mail riviting pliers and I'm quickly getting used to working with mail again, if there's any interest I can host a "workshop" at my house or something.